13-Week Cash Flow Forecast
Find the week your cash gets tightest — before it happens.
Replaces: CFO consulting engagements and $200 spreadsheet templates
How to use it
Enter what you hold today, what you expect to collect, and what leaves on a schedule. The forecast walks forward one week at a time and reports the lowest point rather than the ending balance — because the week you run short is almost never the last one.
Where the number comes from
- Each week opens with the previous week's closing balance, adds collections, and subtracts whatever is due that week.
- Payroll lands on its own cycle rather than being averaged. Averaging is what makes a forecast look survivable when the actual week is not.
- Fixed monthly costs are charged every fourth week, so a 13-week window contains three of them.
- Weekly change in collections compounds: a 2% weekly decline is roughly 23% down by week 13, not 2%.
- The buffer is compared against each week's closing balance, and the largest gap is reported.
What goes wrong
The part most calculators leave out.
- Collections are the number people get wrong. Invoices issued are not cash; a customer who pays at 45 days when you assumed 30 moves two weeks of receipts outside this window entirely.
- A 13-week window flatters anything seasonal. If your quiet quarter starts in week 14, this forecast will not show it.
- Undrawn credit is not cash. Including a facility you have not tested makes the lowest point look safe when the covenant may not permit the draw.
- One-off items are the most commonly forgotten: tax payments, annual insurance, equipment deposits. Each one moves the tightest week.
- This is arithmetic on the numbers you supplied. It does not know your business, and a forecast is not a prediction.
Why the ending balance lies
A company holds 250,000 and expects to end the quarter at 215,000 — a mild decline, apparently fine. But payroll of 120,000 falls on weeks 4, 8 and 12, while a 30,000 fixed run also hits every fourth week. In week 12 both land together against a single week of collections. The closing balance never looks alarming; the week-12 trough sits far below the buffer. The lowest point is the number that governs, and it is invisible in a quarterly view.
Questions
- Why 13 weeks and not three months?
- Thirteen weeks is one quarter expressed in payment cycles rather than calendar months. Payroll, supplier terms and collections all run on weeks, so a weekly grid shows the troughs that a monthly grid averages away.
- Should I use invoices issued or cash collected?
- Cash collected. A 13-week cash forecast is about the bank balance. Invoiced revenue belongs in a different model — this one only cares about the day money arrives.
- What should my minimum buffer be?
- A common starting point is one full payroll run plus fixed monthly costs, so a single late payment cannot make you miss payroll. Businesses with lumpy collections usually hold more.
- Does anything I type get sent anywhere?
- No. The whole calculation runs in your browser. Nothing is transmitted, stored, or logged, and there is no account to create.
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