13-Week Cash Flow Calculator
Will I run out of cash in the next 13 weeks?
Lay out what comes in and goes out week by week and find the lowest week. In the sample, cash stays above zero but falls under the $15,000 reserve in week 5; its lowest point is $14,100 in week 7, $900 short of the reserve. Week 13 closes at $22,400.
Monthly averages hide the weeks that hurt: two payrolls in one month, a tax payment, a customer who pays late. Banks, turnaround advisers and careful owners use a 13-week cash flow forecast for exactly that reason. This one builds the whole quarter from a few weekly numbers, your payroll schedule and one big payment and receipt — and shows the first week that goes wrong.
TightCash stays above zero but falls under your reserve in week 5. The lowest point is $14,100 in week 7 — $900 short of the reserve.
| Lowest cash | $14,100 | at the end of week 7 |
|---|---|---|
| First week under your reserve | Week 5 | your reserve is $15,000 |
| First week under zero | Not in 13 weeks | the week the account would be overdrawn |
| Extra cash needed to keep the reserve | $900 | or that much less going out before the lowest week |
| Cash at the end of week 13 | $22,400 | down $19,600 over the 13 weeks |
| Pay runs in 13 weeks | 7 | in weeks 1, 3, 5, 7, 9, 11, 13 |
| Week | In | Out | Closing cash | Versus reserve |
|---|---|---|---|---|
| 1 | $17,500 | $28,200 | $31,300 | +$16,300 |
| 2 | $17,500 | $7,200 | $41,600 | +$26,600 |
| 3 | $17,500 | $28,200 | $30,900 | +$15,900 |
| 4 | $17,500 | $7,200 | $41,200 | +$26,200 |
| 5 | $17,500 | $44,200 | $14,500 | -$500 |
| 6 | $17,500 | $7,200 | $24,800 | +$9,800 |
| 7 | $17,500 | $28,200 | $14,100 | -$900 |
| 8 | $27,000 | $7,200 | $33,900 | +$18,900 |
| 9 | $17,500 | $28,200 | $23,200 | +$8,200 |
| 10 | $17,500 | $7,200 | $33,500 | +$18,500 |
| 11 | $17,500 | $28,200 | $22,800 | +$7,800 |
| 12 | $17,500 | $7,200 | $33,100 | +$18,100 |
| 13 | $17,500 | $28,200 | $22,400 | +$7,400 |
Runs in your browser with fixed, tested formulas — the same exact arithmetic as the Business X-Ray. Nothing you type is sent or stored.
One question answered. Eight more are waiting.
This page tests one thing with a few monthly numbers. The Business Decision X-Ray runs your whole business through the same engine, day by day: nine tests, the exact breakpoint, the smallest repair and a dated action plan. Open demo of every section, no sign-up.
How this calculator works
Each week starts with last week’s closing cash, adds the money coming in and takes away the money going out. Money in is your weekly average plus the one large receipt in its week. Money out is your weekly average, plus payroll in a pay-run week, plus the one large payment in its week.
Pay runs: every week from the first pay-run week; every two weeks from it; or monthly — the first pay-run week and then 4 or 5 weeks apart (52 ÷ 12 weeks, rounded), which gives three monthly payrolls in 13 weeks. The table shows every week, and the results pick out the lowest week, the first week under the reserve and under zero, and how much extra cash would keep the lowest week at the reserve.
The formula
Closing cashₖ = closing cashₖ₋₁ + receiptsₖ − paymentsₖ, starting from the cash in the bank at the start of week 1.
Receiptsₖ = weekly receipts (+ the one-off receipt in its week). Paymentsₖ = weekly payments (+ payroll on a pay-run week) (+ the one-off payment in its week).
Extra cash needed = max(0, reserve − lowest closing cash).
Worked example
The sample business starts with $42,000, keeps a $15,000 reserve, takes in about $17,500 a week and spends $7,200 a week before payroll. Its $21,000 payroll runs every two weeks from week 1 — 7 times in the quarter — and a $16,000 tax payment is due in week 5; a $9,500 invoice should arrive in week 8.
Payroll weeks lose cash and the weeks between win it back, a little short each time. The tax payment lands in a payroll week, so cash falls under the reserve in week 5, and the lowest point is $14,100 in week 7. $900 more in the bank keeps the reserve intact — and so would a tax payment that falls after the $9,500 invoice arrives. The quarter ends at $22,400.
What this calculator does not do
- It uses one average for weekly receipts and payments, one payroll amount and one extra payment and receipt. Seasonal weeks and several dated bills need the Business X-Ray.
- Weeks are counted from today; months are approximated as 52 ÷ 12 weeks for a monthly payroll.
- It shows cash only — not profit, tax owed or what is still to be invoiced. It is not accounting or financial advice.
Questions people ask
Why 13 weeks?
Thirteen weeks is one quarter: long enough to include every payroll, rent and tax date, short enough that the weekly figures are still known rather than guessed. It is the standard horizon lenders and advisers ask for when cash is tight.
What should go into the weekly averages?
Money actually arriving and leaving the bank, not sales and expenses: customer payments received, and supplier bills, rent, software, loan payments and owner draws paid. Leave out payroll and the one-off items — they have their own fields.
I have more than one big payment. What do I do?
Add the smaller regular ones to the weekly average and enter the largest as the one-off. For a full list of dated payments and invoices, the Business X-Ray builds the day-by-day path.
Should I update it every week?
Yes — that is how 13-week forecasts are meant to be used. Each week, enter the new opening cash and roll the one-off items forward; the page recalculates as you type.
Related decisions
- Cash Runway Calculator — How many months your cash lasts — now, with no sales, and if sales fall.
- Deal Safety Calculator — Whether a big order keeps your cash safe until the customer pays.
- Business Loan & Cash Advance Cost Calculator — The true APR of a merchant cash advance or loan, from its payments.
- How to stress test your small business cash flow
- How long can my business survive without sales?
Formulas reviewed 26 September 2026
