Profitable businesses still fail when they run out of cash at the wrong moment. A cash flow forecast is the simplest tool you have for seeing trouble coming, and it does not require fancy software or an accountancy degree. This guide shows you how to build a practical forecast for a UK small business, model a few scenarios and act before a shortfall becomes a crisis.
Why forecast cash flow
A forecast answers one blunt question: will there be enough money in the bank to meet your obligations on each date they fall due? Wages, rent, supplier invoices, VAT and tax payments all land on fixed days, while your income often arrives late and unevenly. Forecasting lets you spot the gap between the two in advance, so you can chase invoices, delay a purchase, or arrange finance calmly rather than under pressure.
It also makes better decisions possible. Whether you can afford to hire, buy equipment or take on a large order all come down to timing. A good forecast turns a gut feeling into a number you can trust.
Cash flow versus profit
This distinction trips up many owners. Profit is income minus expenses over a period, recorded when earned or incurred. Cash flow is money actually moving in and out of your bank account, recorded when it lands. The two rarely match.
You can be highly profitable on paper yet unable to pay this week’s wages, because:
- A customer has been invoiced but has not paid yet.
- You have bought stock that has not sold.
- A VAT or Corporation Tax bill is due before the related income arrives.
If you would like the underlying theory, see our guide to accounting principles. For day-to-day management, though, cash is what keeps the lights on.
Building a rolling 13-week forecast
A rolling 13-week forecast is the practical standard for small businesses. Thirteen weeks is roughly a quarter, far enough to see VAT and tax bills approaching, near enough to estimate with confidence. “Rolling” means each week you drop the week just finished and add a new week at the end, so you always look a full quarter ahead.
The structure is straightforward:
| Row | What it shows |
|---|---|
| Opening balance | Bank balance at the start of the week |
| Cash inflows | Customer receipts, grants, loans, refunds |
| Cash outflows | Wages, suppliers, rent, VAT, tax, loan repayments |
| Net movement | Inflows minus outflows for the week |
| Closing balance | Opening balance plus net movement |
The closing balance of one week becomes the opening balance of the next. The single most important thing to watch is whether any closing balance turns negative — that is the week you will run short.
Estimating inflows and outflows
Forecasting is informed estimation, not fortune-telling. Build each side from what you already know:
- Inflows: start with your sales ledger. List unpaid invoices by their realistic payment date, not the due date, because regular late payers will be late again. Add expected new sales based on your pipeline and recent averages.
- Outflows: most are predictable. Rent, salaries, software subscriptions and loan repayments recur on known dates. Add variable costs such as stock and materials, and do not forget irregular but certain items like insurance renewals.
Be deliberately cautious: assume income arrives a little later and costs a little higher than hoped. Tightening your terms helps too — see setting payment terms and our advice on dealing with late payers.
Timing of VAT and tax
Tax payments are the outflows most likely to wreck an otherwise healthy forecast, because they are large, occasional and easy to overlook.
- VAT is usually paid quarterly, roughly a month and a week after each quarter ends. The amount you owe is largely money you have already collected from customers, so it should never be treated as your own.
- Corporation Tax for a limited company is due months after the year end, and Self Assessment for a sole trader has its own deadlines, sometimes with payments on account.
Plot every one of these on your forecast on the date the cash actually leaves your account. The cleanest defence is to ring-fence the money as it comes in — read setting aside money for tax and, for VAT specifically, managing VAT cash flow.
Best, worst and likely scenarios
A single forecast gives false comfort. Build three versions from the same template:
- Likely — your honest base case using realistic timings.
- Worst — a key customer pays a month late, a big sale slips, or a cost overruns.
- Best — invoices clear promptly and the pipeline converts well.
The worst case matters most. If it shows a shortfall, you have time to act: arrange an overdraft, defer discretionary spending or push collections harder. Adjusting one assumption — say, paying a supplier a week later — and watching the closing balances respond is the whole point of the exercise.
Acting on a forecast
A forecast only earns its keep if it changes what you do. When a negative week appears, your levers are:
- Pull cash in sooner: invoice promptly, ask for deposits, offer card payment, chase overdue accounts.
- Push cash out later: negotiate supplier terms, time large purchases for stronger weeks, spread payments where possible.
- Add a buffer: an agreed overdraft or invoice finance bridges short, predictable gaps.
Many of these come down to working capital — the cash tied up in stock, debtors and creditors. Our guide to improving working capital covers the structural fixes that make every future forecast healthier.
Tools to use
You do not need much to start:
- A spreadsheet is perfectly adequate for most small businesses and forces you to understand the numbers.
- Accounting software can pull live invoice and bank data and project forward automatically, saving time as you grow and supporting Making Tax Digital record-keeping. See choosing accounting software.
- Open Banking feeds keep your opening balance accurate without manual entry.
Whichever you choose, the discipline matters more than the tool: review the forecast weekly, compare it against what actually happened, and refine your assumptions each time.