A 13-week cash flow forecast is a week-by-week projection of every pound coming into and going out of your business over the next quarter, giving you a clear view of when you will have cash and when you might run short.
You do not need accounting software or a finance team to build one. A spreadsheet is all it takes.
Why 13 Weeks?
Thirteen weeks is one quarter – a time frame that hits a sweet spot. It is far enough ahead to see a cash shortfall coming with time to act – chase invoices, delay a purchase, arrange a working capital finance facility. And it is close enough that most of your numbers are based on invoices already raised, orders placed, and commitments made, rather than guesswork.
Banks and turnaround specialists use this format. If you ever need to discuss cash with a lender, arriving with a 13-week forecast signals that you understand your business.
The Structure
Your forecast has seven rows that repeat for each of the 13 weeks:
| Row | What It Shows |
|---|---|
| Opening balance | Cash in the bank at the start of the week |
| Cash receipts | Money you expect to receive that week |
| Total cash in | Sum of all receipts |
| Cash payments | Money going out that week |
| Total cash out | Sum of all payments |
| Net cash flow | Total cash in minus total cash out |
| Closing balance | Opening balance plus net cash flow |
The closing balance of each week becomes the opening balance of the next. That chain of balances is the core of the forecast – it shows you exactly when your balance dips low and when it recovers.
Step-by-Step: Building Your Forecast
Step 1: Set Up the Spreadsheet
Create 14 columns: one for row labels and one for each of the 13 weeks. Label each week with its start date. Put your actual bank balance in the opening balance cell of week one.
Step 2: Map Out Your Cash Receipts
This is where most forecasts go wrong, so be careful.
Use expected payment dates, not invoice dates. If your average Days Sales Outstanding is 42 days, an invoice raised today will not generate cash for roughly six weeks. An invoice raised three weeks ago should arrive in about three more weeks. This distinction between invoicing and collection is critical – if you use invoice dates, your forecast will be too optimistic and the whole exercise is wasted.
Break receipts into categories that make sense for your business:
- Customer payments – the main one for most businesses. Work from your aged debtor report and your pipeline of expected invoices.
- Recurring revenue – subscriptions, retainers, or contracts with predictable payment schedules.
- Other income – VAT refunds, grant payments, asset sales, tax rebates, anything else.
For invoiced payments, estimate arrival dates based on your experience with each customer. For future sales not yet invoiced, make a reasonable estimate and flag it as an assumption.
Step 3: Map Out Your Cash Payments
Group your outgoings into categories:
Fixed costs (predictable):
– Rent and rates
– Salaries and wages (including employer NI and pension contributions)
– Insurance premiums
– Loan and lease repayments
– Software subscriptions and standing orders
Variable costs:
– Supplier payments (materials, stock, subcontractors)
– Utilities
– Marketing and advertising spend
– Travel and expenses
Tax payments:
– VAT (quarterly – know your next payment date and estimate the amount)
– PAYE and NI (monthly, due by the 22nd if paying electronically)
– Corporation tax (note the due date – typically nine months and one day after your accounting year end)
Capital expenditure:
– Any planned equipment purchases, vehicle costs, or significant one-off investments
Do not forget items that come up less frequently – annual insurance renewals, quarterly professional subscriptions, or biannual software licences. These irregular payments are the ones that catch businesses off guard.
Step 4: Calculate the Weekly Balances
For each week, start with the opening balance (previous week’s closing balance, or your actual bank balance for week one), add total cash in, and subtract total cash out. The result is your closing balance. Scan across all 13 weeks – if any closing balance drops below zero or below your safe buffer, you have identified a potential shortfall.
Step 5: Add Scenarios
A single forecast gives you one version of the future. Three give you a range:
- Expected case – your best estimate of what will actually happen.
- Best case – customers pay on time, that new contract lands.
- Worst case – your largest customer pays late, a major expense arrives early, the new contract gets delayed.
Start with expected, then copy and adjust the key variables. The worst case is particularly valuable – it tells you the minimum cash buffer you need.
How to Use Your Forecast
Review Weekly
Set a recurring 30-minute slot. Compare actuals against your forecast and update the next 13 weeks. This rolling approach means you always have a quarter of visibility ahead.
Roll It Forward
When week one becomes history, add a new week 13 at the end. Moving actuals into the forecast lets you see where predictions were right and where they were off. Over time, this makes your forecasting significantly more accurate.
Share It
If you have a business loan or overdraft, sharing your forecast proactively during a bank review meeting builds confidence and makes conversations about finance options smoother. Your accountant should see it too – they can spot issues you have missed, particularly around tax timing.
Common Mistakes
Using accrual figures instead of cash. Your P&L records revenue when earned, not when collected. Your forecast must use actual cash movement dates. Copy P&L numbers and you will overstate receipts.
Forgetting VAT timing. If customers have not paid you yet, you are still liable to HMRC for the VAT. Many businesses get caught paying VAT before they have collected it. Map your VAT payment dates carefully.
Being too optimistic about collection speed. If your terms are 30 days but your actual DSO is 45, use 45. Forecast based on how customers actually behave. Check your real collection patterns with the Working Capital Calculator.
Ignoring seasonal patterns. If your business has seasonal cash flow patterns, a forecast built in your busy season will look completely different from a quiet period. Use historical data to calibrate.
Never updating it. A forecast six weeks old gives you false confidence. The value comes from the weekly discipline of comparing forecast to reality.
Getting Started
You do not need to wait until your numbers are perfect. Start with the information you have – your bank balance, your aged debtors, your known commitments – and build from there. The first version will be rough. By the fourth or fifth weekly update, you will have a tool that genuinely helps you see around corners.
The 13-week cash flow forecast will not solve your cash flow problems on its own. But it will show you exactly where those problems are, how big they are, and how much time you have to deal with them. That is worth the hour it takes to build.
This article is for general information only and does not constitute financial advice. Seek professional advice for decisions specific to your business.