Running out of cash is the most common reason UK small businesses fail — not a lack of customers, not excessive overheads, and not even poor products or services. Businesses fail because the money they need is not in their bank account at the moment they need it to pay wages, suppliers, rent, or HMRC.
A cash flow forecast does not prevent bad things from happening. What it does is give you time to see them coming. That time — three weeks, three months, three quarters — is the difference between a problem you can manage and a crisis you cannot.
The evidence is stark. The Chartered Institute of Credit Management found that 82% of UK SMEs have faced cash flow difficulties. In Q1 2026, there were 17.48 million overdue invoices on UK books — up 3% on the same quarter the year before. New legislation announced in March 2026 will cap large-firm payment terms at 60 days and add mandatory interest on late payments, which will help over time — but most SMEs are still waiting 60 to 90 days for invoices to be settled while their own HMRC liabilities, payroll, and suppliers demand payment on schedule.
At CoreAcc Accountants, we build and review cash flow forecasts for clients across Hertfordshire and North London. This guide explains how to build one from scratch — what goes in, how far ahead to look, what to do with the numbers once you have them, and what specifically to plan for in 2026/27 given the payroll cost increases, VAT thresholds, and Making Tax Digital changes currently in play.
Disclaimer: This article is for guidance only and does not constitute financial advice. Always seek specific advice for your individual business circumstances.
1. Cash Flow vs. Profit: The Most Important Distinction in Business Finance
Before building a forecast, it is essential to understand why a cash flow forecast is different from a profit and loss account — and why a profitable business can still run out of cash.
Your profit and loss account records income when you earn it and expenses when you incur them, regardless of when money actually changes hands. If you invoice a client £10,000 today on 60-day payment terms, your P&L shows £10,000 of revenue today. Your bank account shows nothing for two months.
Your cash flow forecast records money when it physically enters or leaves your bank account. That same £10,000 invoice appears in your cash flow in month three — not month one.
The gap between these two is called the cash conversion cycle or the cash gap. For many businesses — particularly those in professional services, construction, recruitment, and wholesale trade — the cash gap is the single biggest financial risk they face. It grows silently as the business expands, because larger businesses typically have larger invoices, longer payment cycles, more suppliers to pay upfront, and more payroll commitments to meet before the cash comes in.
The pattern that sinks growing businesses
The scenario we see most often: a profitable business whose customers pay 60 to 90 days late, while their own suppliers, HMRC quarterly VAT bill, and payroll all require payment on the dot. The business is winning work, invoicing clients, generating profit on paper — and quietly running out of money because its working capital cycle has stretched from 30 days to 75 without anyone noticing.
A cash flow forecast makes this visible months in advance. The question it answers is not "are we profitable?" — your accounts answer that. The question is: "will we have money in the bank on the day we need to pay?" These are entirely different questions, and only a cash flow forecast answers the second one.
2. The Two Forecasts Every Small Business Needs
Most businesses benefit from maintaining two complementary forecasts simultaneously:
The 12-month rolling forecast: strategic visibility
A 12-month rolling cash flow forecast gives you strategic visibility over your business's cash position. It is updated monthly — adding a new month at the far end each time you close off the current one — so you always have a full year's view ahead of you.
This is the forecast your bank or lender will want to see. It is the tool that tells you whether you can afford to hire someone new in September, take on a large contract in January, or repay a loan in the spring without running into a cash shortfall. It is also where your annual tax liabilities — Corporation Tax, Self Assessment, VAT, and PAYE — should be planned for and plotted so they do not arrive as surprises.
The 13-week rolling forecast: operational control
A 13-week (approximately three-month) rolling forecast operates at a much more granular level. It maps expected receipts and payments week by week rather than month by month, and is updated every week — replacing each week's projections with actual figures as they come in, and adding a new week at the far end.
The 13-week model is the early warning system. It is the tool that tells you whether you will have a cash gap in the third week of October. At that level of detail and that frequency of update, you surface problems with enough time to act: chase an invoice, delay a discretionary purchase, or arrange a short-term facility with your bank before the gap arrives.
Businesses with seasonal revenue, slow-paying customers, low working capital reserves, or ongoing HMRC liabilities in particular will benefit from running both forecasts simultaneously.
3. Building Your 12-Month Cash Flow Forecast: Step by Step
Here is the structure of a 12-month cash flow forecast for a typical small limited company. The columns are months (January through December, or whichever 12-month period you are forecasting). The rows fall into three sections: cash in, cash out, and the running balance.
Section A: Cash In (Receipts)
List every source of cash that will enter the business bank account, in the month it is expected to actually arrive — not the month it is invoiced or earned.
The most common error on the receipts side: projecting sales at invoice date rather than payment date.
If your standard payment terms are 60 days and you invoice £20,000 in January, that £20,000 appears in your cash flow in March — not January. Many first-time forecasters miss this and produce a forecast that looks comfortable but actually understates the cash gap by two to three months of sales volume.
Section B: Cash Out (Payments)
List every cash payment leaving the business, in the week or month it actually leaves your bank account.
Section C: The Running Balance
Any month where the closing balance is negative represents a cash shortfall — a month in which your business will not have enough money to meet its commitments unless action is taken. The earlier you see this in the forecast, the more options you have.
4. The UK Tax Calendar: The Payments Every Forecast Must Include
One of the most common reasons small business cash flows go wrong is failing to plan for HMRC payment dates. Tax payments are large, infrequent, and fixed — precisely the type of payment that creates cash gaps when not planned for.
Every small business cash flow forecast must include:
Corporation Tax
Corporation Tax is due nine months and one day after the end of your company's accounting period. For a company with a 31 March year end, CT is due by 1 January the following year. For a 31 December year end, it is due by 1 October.
Large companies (profits above £1.5 million) pay quarterly instalments in months 7, 10, 13, and 16 of the accounting period. Most small companies pay once, on the nine-month-and-one-day deadline.
Budget: approximately 19–25% of your expected taxable profit, depending on whether you are above or below the £50,000 / £250,000 CT thresholds.
VAT
If your business is VAT registered, VAT returns are submitted quarterly (for most businesses). The payment is due one month and seven days after the end of each quarter:
The VAT liability is the tax you collected on sales minus the VAT you paid on purchases. It is not income — it is money held on HMRC's behalf. Many businesses make the mistake of spending VAT receipts and then struggling to meet the quarterly bill. A VAT reserve account — a separate pot into which you transfer the VAT element of every payment received — eliminates this problem entirely.
PAYE and employer NI
Monthly PAYE and employer NI must be paid to HMRC by the 19th of the following month (22nd electronically). For a business with a monthly payroll of £30,000 gross, the PAYE and NI payment to HMRC might be £7,000–£10,000 per month depending on the tax codes and NI categories involved. This is a fixed, regular outgoing that must appear in every month of your forecast.
Self Assessment (personal tax)
If you are a director with income from salary, dividends, rental income, or other sources, you pay personal tax through Self Assessment. The key payment dates are:
- 31 January: Balancing payment for the prior tax year, plus first payment on account for the current year
- 31 July: Second payment on account for the current year
For a director earning £80,000 in 2026/27, the combined January and July Self Assessment payments may total £15,000–£25,000 or more. These are personal payments — but they affect you personally and therefore affect how much cash you need to extract from the business in advance.
5. The 2026/27 Costs Every Forecast Must Reflect
The specific economic and regulatory environment of 2026/27 introduces several cost increases that must be built into any forecast prepared this year.
Employer National Insurance: 15% from April 2025
The employer NI rate rose from 13.8% to 15% in April 2025, and the secondary threshold — the salary level above which employer NI is payable — fell from £9,100 to £5,000 per year. For a business with a payroll of £300,000, this change adds approximately £15,000 per year to employer NI costs compared to the 2024/25 position. Every 2026/27 forecast must use the 15% rate above the £5,000 threshold.
National Living and Minimum Wage increases: April 2026
From 6 April 2026, the National Living Wage for workers aged 21 and over rose from £12.21 to £12.71 per hour — a 4.1% increase. The rate for 18 to 20-year-olds rose from £10.00 to £10.85 — an 8.5% increase. For labour-intensive businesses in retail, hospitality, care, and cleaning, these increases feed directly into monthly payroll costs and must be reflected in every month of the forecast from April 2026 onwards.
VAT registration threshold: £90,000
If your business's rolling 12-month turnover approaches or crosses £90,000, you must register for VAT within 30 days. VAT registration changes your cash flow significantly: you start collecting VAT on sales (which improves short-term cash receipts but creates a quarterly payment obligation) and reclaiming VAT on purchases (which reduces your effective cost of inputs). Model a VAT registration scenario in your forecast if your turnover is anywhere near the threshold.
Making Tax Digital for Income Tax: April 2026 onwards
From 6 April 2026, sole traders and landlords with gross income above £50,000 must maintain digital records and submit quarterly updates to HMRC using MTD-compatible software. The quarterly update deadlines fall on 5 August, 5 November, 5 February, and 5 May. These are not payment deadlines — no payment is required at the quarterly update stage — but the requirement to maintain real-time digital records means your accounting data should be current enough to feed your cash flow forecast automatically if you are using MTD-compatible software.
For limited companies, Making Tax Digital does not currently apply to Corporation Tax filing, though MTD for CT remains on the government's long-term roadmap.
6. Worked Examples: Cash Flow in Practice
Worked Example 1: The Cash Gap That Catches Service Businesses
ProServe Consulting Ltd is a small IT consultancy. It has monthly revenue of £60,000 (all invoiced at the start of the month on 60-day payment terms), monthly staff costs of £28,000, and other monthly overheads of £12,000. At first glance it looks profitable: £60,000 revenue minus £40,000 costs = £20,000 profit per month.
But the cash flow tells a different story:
Starting from zero, ProServe would be £80,000 overdrawn in February before a single pound of profit materialises. Without a £80,000 working capital facility or cash reserve, it cannot pay its staff or overheads in January and February — even though it is profitable, growing, and has invoices outstanding that will eventually be paid.
A cash flow forecast produced in December would have made this visible in time to arrange an overdraft facility or restructure the payment terms with clients before the gap arrived.
Worked Example 2: Planning for the VAT Cliff Edge
Brightline Design Ltd is a branding agency. In March 2026, its cumulative 12-month rolling turnover was £84,000. The business is not yet VAT registered. Its founder projects £10,000 of new work per month for the rest of the year.
A cash flow forecast built in March 2026 shows:
Brightline crosses the £90,000 VAT threshold in April 2026. It must register within 30 days and start charging VAT on its invoices from the date of registration. Its first VAT return will cover the quarter ending June 2026 (£30,000 of invoices × 20% = £6,000 VAT due to HMRC on 7 August 2026).
Without a forecast, this £6,000 payment arrives as a surprise in August. With the forecast, the founder has known since March that it was coming, has set aside the VAT collected each month, and pays it on time without any cash impact.
Worked Example 3: The Quarterly Tax Payment Plan
Oakfield Recruitment Ltd has a 31 March year end. Its expected taxable profit for the year ending 31 March 2026 is £85,000. Corporation Tax at 25% marginal rate: approximately £18,500, due by 1 January 2027.
The company also has a quarterly VAT liability of approximately £9,500 per quarter, due in May, August, November, and February.
And the sole director draws dividends of £80,000 per year, creating a Self Assessment liability: first payment on account of approximately £10,000 due 31 January 2027, second payment on account 31 July 2027.
The forecast maps all of these:
Total major tax payments between May 2026 and January 2027: £83,500. Spread across nine months that averages £9,278 per month — but the actual payment dates are lumpy, with January 2027 alone requiring £37,500 in two separate payments (CT and Self Assessment).
A business that has not built this into its forecast will face a severe January cash crunch despite being profitable all year. A business with this mapped out starts saving for January in April.
Worked Example 4: Three-Scenario Modelling
Ridge & Sons is a small building contractor with highly seasonal revenue — 60% of its income falls in the April-to-September period. In March 2026 it builds a three-scenario 12-month forecast:
Base case: Revenue broadly in line with 2025/26. New contracts begin in April as expected.
Best case: A quoted commercial refurbishment contract (£120,000, quoted in February) is confirmed in April and paid in three instalments: £40,000 on commencement, £40,000 at practical completion, £40,000 on final account. This boosts cash significantly in April, August, and November.
Worst case: The commercial contract is delayed until September. A key subcontractor is unavailable from June, reducing output by 20% for three months. Energy cost for plant increases by 15%.
The three forecasts show:
The worst case shows a cash shortfall of £14,200 in October. Ridge & Sons contacts their bank in March and agrees a £20,000 seasonal overdraft facility — available if needed, cost-free if unused. In the event, the commercial contract is delayed and October does produce a £11,600 shortfall. The overdraft covers it. The business continues trading.
Without the three-scenario forecast, Ridge & Sons would have had no overdraft in place and would have faced a cash crisis — despite having profitable contracts underway and a healthy order book.
7. Sensitivity Analysis: Finding Out Where Your Forecast Is Most Fragile
Three-scenario modelling tells you what a specific set of events looks like — a named best or worst case with multiple variables changed simultaneously. Sensitivity analysis is a different and complementary tool: it changes one variable at a time to identify which single parameter has the greatest impact on your closing cash balance. The distinction matters because it answers a different question. Scenario analysis says "what if we lose the contract and a subcontractor becomes unavailable?" Sensitivity analysis says "of all the things that could go wrong, which one hurts us the most?"
This is the technique used by lenders, investors, and experienced FDs when stress-testing a forecast. It is also one of the most practically useful exercises a small business owner can run — because it tells you exactly where to focus your management attention and risk mitigation effort.
How to run a sensitivity analysis
Take your base case forecast. Then change each key variable individually, by a realistic but meaningful amount — typically 5%, 10%, or 20% depending on the variable — while holding everything else constant. Note the impact on your lowest monthly closing balance and on cumulative cash flow over the forecast period. The variable that produces the largest negative movement is your primary sensitivity — the thing your business is most exposed to and that deserves the most active management.
The six parameters worth testing for most small businesses are:
Parameter 1: Debtor days (average payment time)
For any business selling on credit terms, this is almost always the highest-impact variable. A movement in average payment time from 45 days to 60 days means every sale lands in your bank two weeks later than planned. Across a month's worth of invoicing, that is two weeks of revenue that disappears from your near-term cash position.
Test: What happens to my closing balance if my average debtor days move from 45 to 60? From 60 to 75?
Parameter 2: Revenue volume
What does a 10% or 20% reduction in sales do to your monthly closing balance? This is particularly important for businesses with high fixed costs — where a 15% fall in revenue does not mean a 15% fall in profit, because salaries, rent, and loan repayments do not reduce proportionally. The sensitivity analysis makes the break-even point visible and tells you how much revenue you can afford to lose before the business hits a cash problem.
Test: What happens to my closing balance if revenue falls 10%? 20%?
Parameter 3: Gross margin
For product-based businesses, a change in the cost of goods sold — materials, supplier prices, import costs — directly erodes margin without any change in sales volume. A business running at 40% gross margin that sees supplier costs rise by 8% is effectively running at 35% gross margin on the same turnover, with a direct hit to the cash available to cover overheads.
Test: What happens to my closing balance if my cost of sales increases by 5%? 10%?
Parameter 4: Payroll headcount
The decision to hire — or the risk of an unexpected resignation requiring a replacement — has an immediate and specific cash flow impact. A sensitivity analysis on headcount makes the exact monthly cost of one additional hire visible: salary, employer NI at 15% above £5,000, pension contribution, and any equipment or onboarding cost. It also shows how many months it typically takes for revenue growth to cover that cost.
Test: What happens to my closing balance if I add one member of staff at £35,000 per year in month four?
Parameter 5: Interest rates
With the Bank Rate currently at 3.75% and the next MPC decision due 30 July 2026, businesses with variable-rate borrowing face genuine near-term uncertainty. A sensitivity test on interest rates shows what a 0.5% or 1% move in base rate does to your monthly loan or overdraft cost — and therefore to your closing balance over the forecast period.
Test: What happens to my monthly financing cost if the Bank Rate rises by 0.5%? By 1%?
Parameter 6: Key customer concentration
If a single customer accounts for more than 20% of your revenue, their behaviour is a material risk to your forecast. A sensitivity test models two scenarios: they pay 30 days late, and they stop paying entirely (insolvency or dispute). This is not a comfortable exercise — but it is the test that tells you whether your business is resilient to the loss of its biggest client, or whether that single relationship is an existential dependency that deserves active management.
Test: What happens to my closing balance if my largest customer pays 30 days late for three months? What if they stop paying entirely?
Worked Example 5: Sensitivity Analysis for a Service Business
Clearwater Consulting Ltd has annual revenue of £480,000 (£40,000 per month), a gross margin of 65%, and fixed monthly overheads of £18,000. Its base case closing balance at its lowest point (month nine) is £24,500.
The directors run a sensitivity analysis on four variables, changing each one individually:
The sensitivity analysis tells Clearwater's directors something important: debtor days is their biggest single risk — a 15-day increase in average payment time reduces their minimum closing balance from £24,500 to just £6,200, close to a position where a single unexpected cost could put them in the red.
Revenue volume is the second most significant variable. Gross margin matters but is less impactful than payment behaviour. Interest rate movement is almost negligible given the relatively modest facility.
The practical response: Clearwater decides to introduce a stricter credit control policy — formal credit checks on new clients, 14-day payment terms for clients under 12 months old, and a weekly aged debtor review. These are not dramatic measures, but the sensitivity analysis has shown precisely why they are the most important thing the business can do to protect its cash position — more impactful than cost-cutting, refinancing, or margin improvement.
Sensitivity analysis vs. scenario analysis: using both together
The most complete approach is to use sensitivity analysis to identify your primary risk variables, then build your worst-case scenario around the combination of those variables deteriorating simultaneously. Clearwater's worst case, informed by the sensitivity analysis, combines debtor days moving to 60 days AND revenue falling 10% — the two highest-impact variables. That combined scenario produces a month-nine closing balance of negative £12,200, which tells the directors exactly how large a facility or cash reserve they need to hold as a buffer.
Running the sensitivity analysis first means the worst-case scenario is grounded in the specific vulnerabilities of the business rather than being a generic "things go badly" model. That makes it far more credible to a bank, an investor, or HMRC — and far more useful as a management tool.
8. The Thirteen Most Common Cash Flow Mistakes
1. Using invoice date instead of payment date for receipts. Project cash when it arrives in your bank, not when you raise the invoice.
2. Ignoring tax payment dates. Corporation Tax, VAT, PAYE, and Self Assessment are large, infrequent payments. Map every one with its exact due date.
3. Treating VAT receipts as income. VAT collected from customers is not your money. It is held on HMRC's behalf and must be paid over quarterly. Keep it in a separate mental (or actual) pot.
4. Over-optimistic revenue projections. The most common first-time forecasting error. Use your actual historical payment patterns, not your best-case hopes. If 40% of your invoices are paid late, model that.
5. Failing to account for seasonality. Most businesses are not flat month-to-month. Build your forecast using realistic monthly revenue profiles based on prior year patterns.
6. Omitting one-off capital expenditure. A new van, a piece of machinery, or a website build can cost £10,000–£50,000. These appear once in the forecast but have significant impact on the month they hit.
7. Not tracking days sales outstanding (DSO). DSO is the average number of days your customers take to pay. If DSO creeps from 35 to 55 days, your cash gap grows by 20 days of average daily sales — a significant number for any business above £500,000 turnover.
8. Ignoring personal tax liabilities. As a director, your Self Assessment tax bill is not a company cost — but you need money from the company to pay it. If you have not extracted enough cash during the year, January becomes painful.
9. Not updating the forecast. A cash flow forecast that was built in January and not touched since is almost useless by April. Update monthly at minimum — weekly if cash is tight.
10. Planning only the base case. A forecast without a worst case does not prepare you for the scenarios that actually test the business. Always build at least three versions.
11. Forgetting the employer NI increase. From April 2025, employer NI is 15% above a £5,000 threshold. Any forecast using the old 13.8% rate above £9,100 is understating payroll costs materially.
12. Using manual spreadsheets disconnected from live data. A forecast built from a spreadsheet you update by hand is only as good as your memory. MTD-compatible accounting software (Xero, QuickBooks) feeds live transaction data into your forecast automatically, removing the lag between reality and your projections.
13. Not acting on what the forecast shows. A forecast that predicts a cash gap in three months is only valuable if you use those three months to act — chase invoices, arrange finance, delay discretionary spending, or accelerate receipts. The forecast is an early warning system, not a planning document to file and forget.
9. What to Do When the Forecast Shows a Shortfall
If your cash flow forecast shows a negative closing balance in any future month, you have time to act. The options available to you depend on how far in advance you spotted the problem.
Accelerate cash coming in
- Chase outstanding invoices — Days Sales Outstanding is frequently the biggest lever available
- Offer a small early payment discount (for example, 1.5% for payment within 7 days) to incentivise faster settlement
- Move from monthly to weekly invoicing for project-based work
- Request stage payments or deposits on larger contracts rather than payment at completion
- Review your payment terms — if 60 days is standard in your industry but 30 days is feasible, renegotiate
Delay or reduce cash going out
- Negotiate extended payment terms with key suppliers — most suppliers would rather wait an extra 30 days than lose a customer
- Defer non-essential capital expenditure to a later period
- Review discretionary overheads — subscriptions, marketing spend, training — that can be paused temporarily
- Consider salary sacrifice arrangements (pension, EV) that reduce gross payroll cost without reducing take-home pay for staff
Arrange external finance
The most appropriate financing tool depends on the nature and duration of the shortfall:
Business overdraft: For short-term, recurring cash gaps — bridging the gap between invoicing and receipt. Low cost, flexible, but requires lender comfort on the business's overall credit position.
Invoice finance (factoring or invoice discounting): Releases a proportion (typically 70–90%) of an invoice's value immediately on issue, rather than waiting for the customer to pay. Very effective for businesses with long payment cycles and creditworthy debtors. Invoice discounting is confidential (your customers do not know you are using it); factoring typically involves the finance company managing your debtor ledger.
Asset finance: For capital expenditure — vehicles, equipment, machinery — spreading the cost over 2–5 years rather than one lump sum. Preserves cash for working capital.
Recovery Loan Scheme: A government-backed scheme providing term loans and asset finance for SMEs. Check current availability as the scheme's terms and the government guarantee percentage change periodically.
HMRC Time to Pay: If your cash gap is driven by an inability to pay a tax liability on time, HMRC's Time to Pay arrangement allows you to spread payments over time — typically up to 12 months. HMRC assesses applications on three criteria: your compliance history, whether the repayment plan is realistic, and whether you contacted them before or after enforcement action started. A cash flow forecast showing exactly how you will meet the instalments significantly increases the likelihood of approval. Contact HMRC before the payment deadline — not after.
Frequently Asked Questions
What is a cash flow forecast and why does my business need one?
A cash flow forecast maps the money expected to enter and leave your business bank account over a future period — week by week or month by month. Unlike a profit and loss account, which records income when earned and expenses when incurred regardless of when cash moves, a forecast shows when money will physically be available. A business can be profitable and still run out of cash if its customers pay late, tax liabilities arrive in lumps, or capital expenditure is poorly timed. A cash flow forecast makes these risks visible in advance, giving you time to arrange finance, accelerate receipts, or delay spending before a shortfall becomes a crisis.
What is the difference between a cash flow forecast and a budget?
A budget sets targets for income and expenditure over a period — it is a planning tool that tells you what you want to achieve. A cash flow forecast models when money will physically arrive and leave your bank account — it is a liquidity tool that tells you whether you will have enough cash available when you need it. A budget answers "are we on track?"; a forecast answers "will we have money to pay next month's wages?" Once you have employees, debt, stock, VAT, or deferred income, a budget alone is not sufficient. A cash flow forecast working alongside it is essential.
How far ahead should I forecast?
Maintain two forecasts simultaneously: a 12-month rolling strategic forecast, updated monthly, for visibility over major commitments and lender requirements; and a 13-week (three-month) rolling operational forecast, updated weekly, for granular visibility over upcoming cash gaps. The 13-week model is updated every week — actual figures replace projections as they come in, and a new week is added at the far end. Businesses with tight working capital, seasonal revenue, or significant debtor exposure should treat the 13-week forecast as a priority management tool.
What is the biggest mistake businesses make in their cash flow forecasts?
Using invoice date rather than payment date for sales receipts is the single most common error. If you invoice £50,000 in January on 60-day payment terms, that cash appears in March — not January. A forecast that shows January receipts of £50,000 when the cash will not arrive until March understates the January and February cash gap and may give false comfort about the business's position. Always base receipt projections on when you realistically expect customers to pay, using historical payment data if available.
How do I include tax payments in my cash flow forecast?
Plot every significant tax payment on its actual due date: PAYE and employer NI by the 19th of the following month; VAT returns one month and seven days after the end of each quarter; Corporation Tax nine months and one day after your company's year end; Self Assessment balancing payments and payments on account on 31 January and 31 July each year. For most small limited companies, the January and October/November period (when Corporation Tax and Self Assessment often fall together) is the most cash-intensive period of the year — plan for it from the beginning of the tax year, not when the bills arrive.
Should I include VAT in my cash flow forecast?
Yes — but you need to distinguish carefully between VAT collected on sales (which passes through to HMRC quarterly and is not your income) and the net impact of VAT on your cash flow. On the receipts side, if your customers pay you inclusive of VAT, your cash inflows will include VAT. On the payments side, you pay net of VAT to HMRC each quarter. The simplest approach for most small businesses is to include all receipts and payments at the VAT-inclusive figure (i.e. what you actually receive and pay) and add a separate line for the quarterly VAT payment to HMRC. This gives you an accurate picture of actual cash movement without needing to track VAT on every individual transaction.
What is Days Sales Outstanding and why does it matter?
Days Sales Outstanding (DSO) measures the average number of days it takes your customers to pay after you invoice them. It is calculated as: (total outstanding debtors ÷ total credit sales) × number of days in the period. If your DSO creeps from 35 to 55 days as your business grows and your customers take longer to pay, your working capital requirement increases by 20 days of average daily sales. For a business turning over £600,000 per year, that is approximately £33,000 of additional working capital needed. Monitoring DSO monthly and including realistic payment lag assumptions in your forecast prevents this from becoming invisible until it causes a crisis.
What can I do if my forecast shows a cash shortfall?
Act as early as possible. The options available depend on the lead time: chase outstanding invoices and reduce DSO; negotiate extended terms with suppliers; offer early payment discounts to clients; move to shorter invoice cycles; arrange an overdraft facility with your bank; explore invoice finance to unlock cash tied up in debtors; defer non-essential capital expenditure; or contact HMRC to arrange a Time to Pay agreement if a tax payment is creating the shortfall. HMRC Time to Pay is available for Corporation Tax, VAT, PAYE, and Self Assessment — but must be applied for before the payment deadline and is more likely to be approved if accompanied by a credible cash flow forecast.
Do I need specialist software to build a cash flow forecast?
No — a well-structured spreadsheet works perfectly well for a straightforward business. However, cloud accounting software such as Xero and QuickBooks integrates with your live bank and invoice data, which means your forecast is always based on current figures rather than manually entered estimates. This significantly reduces the maintenance burden and improves accuracy. Both Xero and QuickBooks include cash flow forecasting modules or integrate with specialist tools. If you are already required to use MTD-compatible software for income tax or VAT returns, the same software will feed your forecast data automatically.
How does Making Tax Digital affect my cash flow forecast?
Making Tax Digital for Income Tax came into force for sole traders and landlords with gross income above £50,000 from 6 April 2026. MTD requires quarterly digital submissions to HMRC — but these are not payment events. No additional tax is due at the quarterly submission stage; the requirement is simply to submit a summary of income and expenses. The benefit for cash flow forecasting is that MTD-compatible software maintains live transaction records throughout the year, giving you a continuously updated view of income and expenses that feeds directly into any forecasting tool you use. For limited companies, MTD for Corporation Tax remains on HMRC's roadmap but has not yet been implemented.
What is sensitivity analysis and how is it different from scenario modelling?
Scenario modelling changes multiple variables simultaneously to model a named event — for example, a worst case that combines a major contract loss with a subcontractor becoming unavailable. Sensitivity analysis changes one variable at a time to identify which individual parameter has the greatest impact on your cash position. The two techniques are complementary: use sensitivity analysis first to identify your primary risk variables (the ones that move your closing balance the most), then build your worst-case scenario around the combination of those variables deteriorating at the same time. The result is a worst case that is specific to your business's actual vulnerabilities rather than a generic "things go badly" model — and one that is far more credible to a lender or HMRC when you need to present it.
What variables should I test in a sensitivity analysis?
For most small businesses, the six highest-impact variables to test are: debtor days (average payment time from your customers), revenue volume (what a 10–20% fall in sales does to your closing balance), gross margin (the impact of supplier cost increases on cash available to cover overheads), payroll headcount (the monthly cost of an additional hire or unexpected vacancy), interest rates (the impact of a 0.5–1% rate move on variable-rate borrowing costs), and key customer concentration (the impact of your largest customer paying late or not paying at all). Change each one individually while holding everything else constant, record the impact on your minimum closing balance, and rank them by impact. The variable with the largest effect is where you should focus your risk mitigation effort first.
My business is seasonal — how do I forecast cash flow accurately?
Build your monthly revenue projections from prior year data rather than dividing annual revenue equally across 12 months. If 60% of your revenue falls in the April-to-September period, your cash receipts in those months should reflect that pattern — adjusted for any payment lag. Seasonal businesses benefit particularly from three-scenario modelling (base, best, worst case) and should arrange any seasonal overdraft facility well before the lean months arrive, when the bank can see from your forecast that the facility will be repaid once the busy season receipts come in. Plan for your worst case and you will never be caught out by it.
Can my accountant help me build a cash flow forecast?
Yes — and this is one of the most valuable things an accountant can do for a small business beyond compliance work. At CoreAcc, we build 12-month cash flow forecasts as part of our business advisory service, integrate them with your management accounts and tax planning, and flag emerging issues before they become problems. We also help clients prepare cash flow forecasts for bank lending applications, HMRC Time to Pay arrangements, and investment pitches — all of which require a credible, professionally prepared forecast that a lender or HMRC can interrogate.
What CoreAcc Accountants Can Help You With
Building a cash flow forecast is not a one-off event. The most valuable forecasts are the ones that are maintained, updated regularly, and connected to both your live accounting data and your tax planning. At CoreAcc Accountants, we help clients with:
- 12-month rolling forecast preparation: Building your first forecast from scratch or reviewing and improving an existing one
- 13-week operational forecasting: For businesses with tight working capital or seasonal patterns who need weekly visibility
- Three-scenario modelling: Base, best, and worst case plans that prepare you for the range of outcomes your business might face
- Sensitivity analysis: Identifying the single parameters that have the greatest impact on your cash position — so you know exactly where to focus your risk management effort
- Tax payment planning: Mapping every Corporation Tax, VAT, PAYE, and Self Assessment liability against its exact due date so your forecast is complete
- MTD software setup: Getting you onto Xero or QuickBooks so your live accounting data feeds your forecast automatically
- Bank and lender support: Preparing cash flow forecasts in the format required by high-street banks and alternative lenders for overdraft, invoice finance, or term loan applications
- HMRC Time to Pay applications: Preparing the supporting cash flow evidence HMRC requires to approve a payment arrangement
Get in Touch
Whether you need a cash flow forecast built from scratch, want your existing one reviewed, or need support preparing a forecast for a bank or HMRC, CoreAcc Accountants is here to help.
CoreAcc Accountants is an ACCA-accredited firm of Chartered Certified Accountants based in Borehamwood, Hertfordshire. This article was last reviewed in August 2026 and reflects the regulatory and economic environment in force at that date. It does not constitute financial or professional advice. Always seek specific advice for your individual business circumstances.
