How to build a 13-week cash flow forecast for a small business

General information, not advice. This article is general information for UK small businesses. It is not legal, tax or financial advice. Rules and figures change, so check the official guidance listed at the end, or speak to a qualified accountant, before you act. Figures checked October 2026.

Plenty of profitable businesses still run out of cash. A big customer pays late, the VAT bill lands in the same week as payroll, or the annual insurance renewal arrives just after you bought new stock. None of these are surprises if you can see them coming. A 13-week cash flow forecast is the simplest way to see them coming.

Thirteen weeks is one quarter. It is short enough that most of the numbers are things you already know: invoices you have sent, bills you have received, wages you will pay. It is long enough to show a gap while you still have time to do something about it, such as chasing a customer, moving a purchase or talking to your bank.

A yearly budget answers "will we make money this year?". A 13-week forecast answers a sharper question: "in which week could we run short, and by how much?"

• Today's balance on every business bank account (not the figure in your accounting software if it is out of date).
• A list of unpaid customer invoices, with the date you honestly expect each one to be paid.
• A list of supplier bills you owe and when you plan to pay them.
• Your payroll dates and the total cost of each payroll.
• Fixed costs: rent, loan repayments, software, insurance, subscriptions.
• Tax dates that fall inside the next 13 weeks (see below).

Use a spreadsheet with 13 columns, one for each week, starting with the current week. Down the side, list:

1. Opening balance
2. Cash in (customer payments, card takings, other income)
3. Cash out (wages, rent, suppliers, tax, loan repayments, owner drawings)
4. Net movement (cash in minus cash out)
5. Closing balance (opening balance plus net movement)

The closing balance of one week becomes the opening balance of the next.

This is where most forecasts go wrong. An invoice due on the 30th is not cash on the 30th if that customer usually pays three weeks late. Use each customer's real habit, not the payment terms on the invoice. For card or till takings, use the average of the same weeks last year or the last 8 to 12 weeks, and be a little cautious.

If customers pay you late, you have rights. Between businesses, you can usually claim statutory interest of 8% above the Bank of England base rate, plus a fixed recovery fee of £40, £70 or £100 depending on the size of the debt. Most owners use this as a reason to agree firmer terms rather than to send a claim, but it helps to know it exists.

Start with the fixed items: payroll, rent, loans. Then add supplier payments and one-off costs you already know about. Do not forget the lines owners most often miss:

• VAT. If you are VAT registered, the return and payment are usually due one calendar month and 7 days after the end of each VAT period. The VAT you collect is not your money; keep it out of your spending plans.
• PAYE. Electronic payments of PAYE and National Insurance are usually due by the 22nd of the following tax month (the 19th if you pay by cheque).
• Corporation Tax. For most small companies, payment is due 9 months and 1 day after the end of the accounting period.
• Annual bills. Insurance renewals, the ICO data protection fee, accountancy fees and software paid yearly.
• What you take out. Owner drawings or dividends are cash out too.

Decide the lowest balance you are comfortable holding, for example one payroll plus one month of rent. Then look along the closing balance row. The week with the lowest figure is the one that matters. If it drops below your floor, you have found your problem early, which is the whole point.

Here is a short, made-up example for a small service business:

Week 3 holds payroll and a VAT payment together. If this owner's floor is £12,000, week 3 needs attention now, not in three weeks.

• Bring cash in sooner. Send invoices the day the work is done, chase before the due date, offer a simple way to pay, and ask for deposits on larger jobs.
• Move cash out later. Ask suppliers for longer terms before you need them, not after. Delay purchases that can wait.
• Cut or pause. Look for subscriptions and spending that do not earn their keep.
• Talk early. Banks and lenders respond better to a forecast shown four weeks ahead than to a phone call on the day. If a tax bill is the problem, HMRC may agree a payment plan if you cannot pay in full; ask before the deadline.

A forecast is only useful if it is current. Once a week, ideally on the same day:

1. Replace last week's forecast with what actually happened.
2. Note any big difference and why (a late payer, a quiet week, an unplanned repair).
3. Add a new week at the end so you always see 13 weeks ahead.

After a month or two you will learn which lines you are always too hopeful about. That is the most valuable thing a forecast teaches.

The Cashflow page in NAVIZIX builds a 13-week forecast from the transactions you record, the recurring income and costs you confirm, and the invoices and bills you have open. It shows a likely range as well as a single line, so you can see how much the next few weeks could swing. Invoices in NAVIZIX can send reminders before and after the due date, which helps with step 2. A forecast is only as good as the numbers you put in, so review it every week like you would a spreadsheet. See how invoices and reminders work.

• GOV.UK: VAT Returns deadlines
• GOV.UK: Pay employers' PAYE
• GOV.UK: Prepare and file annual accounts for a limited company (Corporation Tax payment and return deadlines)
• GOV.UK: Late commercial payments: charging interest and claiming debt recovery costs
• GOV.UK: If you cannot pay your tax bill on time
• U.S. Small Business Administration: Manage your finances