Profit is not the same as cash. A business can be profitable on paper and still run out of money — because customers pay late, VAT is due before invoices are settled, or a quiet month arrives at the wrong time. A cash flow forecast stops those surprises.
A rolling 13-week forecast is one of the simplest and most useful tools for a small business. It shows, week by week, the money you expect to receive and the money you expect to pay out — so you can see a shortfall coming and do something about it. This guide explains how to build one, step by step.
Why Cash Flow Forecasting Matters
Cash flow forecasting matters because timing is everything. Your profit and loss account tells you whether you made money; your cash flow forecast tells you whether you will have money in the bank when a bill is due. The two are often very different.
A forecast helps you spot a cash shortfall weeks before it happens, decide whether you can afford a purchase or a new member of staff, and plan around big fixed dates such as VAT and PAYE. It also gives you confidence when talking to your bank or suppliers, because you can show them exactly how your cash position will develop.
What You Need Before You Start
You do not need special software to build a 13-week forecast. A spreadsheet is enough for cash flow forecasting. Before you start, gather:
- Your current bank balance, including any overdraft limit.
- A list of invoices you have issued and when customers usually pay them.
- A list of regular outgoings — rent, utilities, insurance, subscriptions and loan repayments.
- Payroll dates, PAYE and VAT deadlines for the next 13 weeks.
- Any one-off payments you expect, such as equipment, tax bills or annual renewals.
Building Your 13-Week Forecast, Step by Step
Step 1 — Start With Your Opening Balance
Put your bank balance at the start of week 1 in the first column. This is your opening balance, and it is the foundation of the whole forecast. If you have an available overdraft, note the limit separately — it is a safety net, not cash in the bank.
Step 2 — List Your Expected Receipts
For each week, list the money you expect to receive: invoices that are due and likely to be paid, cash sales, refunds or grants. Be honest about timing — if a customer usually pays two weeks late, forecast them two weeks late, not on the due date. This is the most common source of forecast error.
Step 3 — List Your Expected Payments
Now list everything you expect to pay in each week: wages, rent, suppliers, utilities, subscriptions, loan repayments and any one-off costs. It is easy to forget small recurring payments, so go through your last three months of bank statements and make sure every regular payment is captured.
Step 4 — Include VAT, PAYE and Payroll Dates
Tax deadlines are often the biggest payments in a small business, so build them in accurately:
- VAT is usually due one calendar month and seven days after the end of each VAT period — for many quarterly businesses, this is a big payment that falls in the same week each quarter.
- PAYE and National Insurance are due by the 22nd of the month if you pay electronically (the 19th if paying by cheque).
- Wages and pension contributions should be included in the week they are actually paid, not the month they relate to.
Step 5 — Work Out the Closing Balance Each Week
For each week, the closing balance is: opening balance + receipts − payments. That closing balance becomes the opening balance of the next week. A simple example:
| Week | Opening balance | Receipts | Payments | Closing balance |
|---|---|---|---|---|
| Week 1 | £12,000 | £8,500 | £7,200 (including wages of £4,000) | £13,300 |
| Week 2 | £13,300 | £6,000 | £9,800 (including VAT of £2,500) | £9,500 |
| Week 3 | £9,500 | £10,000 | £5,400 | £14,100 |
Even this simple example shows how the balance can dip in a week when VAT is due — and that is exactly the sort of pressure point a forecast is meant to reveal.
Step 6 — Review and Roll Forward
A cash flow forecasting model is a living document. Each week, replace the forecast figures with the actual amounts and add a new week at the end, so you always have 13 weeks ahead. This keeps the forecast accurate and turns it into a habit rather than a one-off exercise.
Best Case, Worst Case and What-Ifs
A single cash flow forecasting exercise can be misleading if it assumes everything goes to plan. Try two extra versions:
- Best case: customers pay on time and a large order lands as expected.
- Worst case: your two biggest customers pay two weeks late, and a planned sale slips a month.
If the worst case still keeps you in positive cash, you have breathing room. If it does not, you know you need to act — talk to your bank about a buffer, tighten credit control or delay spending — before the problem arrives.
Reviewing Your Forecast Every Week
Set aside 30 minutes a week to review the cash flow forecasting. Check what was received and paid against what you expected, update the next 13 weeks and note any new one-off items.
Also use the review to learn. If customers consistently pay a week later than you expect, change your assumptions. A forecast that is honest about your real payment patterns is far more useful than one that assumes the best.
Accounting Software That Can Help
You can run a 13-week forecast in a spreadsheet, and for many small businesses that is enough. If you want more automation, most cloud accounting packages now offer cash flow reporting: Xero, QuickBooks and FreeAgent can all produce cash flow forecasts from your invoices and bank feeds, and your accountant can set one up that updates automatically as your records change.
Related reading: Good records make cash flow planning possible — our guide to bookkeeping basics for UK small businesses explains the records to keep and a simple monthly routine.
Frequently Asked Questions
How is a cash flow forecast different from a budget or a profit forecast?
A profit forecast shows income and expenses when they are earned or incurred. A cash flow forecast shows money when it actually moves in and out of your bank account. The difference is timing — and timing is what decides whether bills get paid.
How far ahead should I forecast?
For most small businesses, a rolling 13-week forecast is a good horizon: long enough to see VAT and PAYE deadlines coming, short enough to forecast with reasonable accuracy. Larger businesses often forecast 12 months ahead, but with less detail.
What if my forecast shows my bank balance going negative?
Act early. Chase overdue invoices, delay non-essential spending, ask suppliers about extended terms or talk to your bank about a temporary overdraft. Whatever you do, do it weeks ahead — not the day the balance hits zero.
Do I need accounting software to do this?
No. A simple spreadsheet works well for cash flow forecasting, especially if your income and outgoings are predictable. Software helps as the business grows, because it pulls figures from your bookkeeping automatically and reduces manual errors.
How We Can Help
Cash flow problems are usually timing problems, and timing problems can be planned around — if you spot them early enough. Financial Accounting Firm helps small businesses build a 13-week forecast, review it weekly and take practical steps when the numbers look tight. Contact us to get started.
This article is general information only and should not be taken as personal tax or financial advice. Your position will depend on your own circumstances.
