Cash Flow Forecasting for Small Businesses: A Practical UK Guide

Why profitable businesses run out of cash

Your profit and loss account records a sale when you invoice, not when you're paid. If you invoice on 30-day terms and your customers take 55 days, every pound of growth pulls cash out of the business before it puts any back in. Add VAT quarters, corporation tax, payroll and stock, and a genuinely profitable business can hit a wall.

A cash flow forecast fixes this by tracking money in and out of the bank account by date, not by accounting period.

Building a rolling 13-week forecast

Thirteen weeks — one quarter — is the standard horizon because it covers a full VAT cycle and most payment terms while staying accurate enough to trust. Build it as a simple weekly grid:

• Opening bank balance for week one (the actual figure, from your bank).
• Receipts: invoiced sales by expected payment date, not invoice date. Use each customer's real payment behaviour, not their terms.
• Payroll and PAYE/NI, by the date they leave the account.
• Supplier payments, rent, software and other fixed costs.
• VAT, corporation tax and any loan or finance repayments on their due dates.
• Closing balance, which becomes next week's opening balance.

Update it every week

Each week, drop the completed week, add a new week 13, and replace forecast figures with actuals. Two things then happen: you see problems six to ten weeks out while you can still do something about them, and your forecasting accuracy improves quickly because you're constantly checking predictions against reality.

Reviewing the variance matters as much as the forecast. If receipts came in £8,000 under plan, find out whether that's timing or a lost sale — the response is completely different.

Model the downside

Alongside your base case, run one pessimistic scenario: your largest customer pays 30 days late, or revenue drops 20% for a quarter. If that scenario breaks the bank balance in week seven, you know exactly how much headroom you need to arrange — and you can arrange it early, when lenders are far more willing to help.

Common mistakes

The forecasts that fail usually share the same faults:

• Using invoice dates instead of expected payment dates.
• Forgetting VAT and corporation tax, which are the two biggest surprise outflows for UK SMEs.
• Assuming everyone pays on terms.
• Building it once and never updating it.
• Forecasting profit instead of cash, and treating the two as interchangeable.

Frequently asked questions

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