13 Week Cash Flow Starter

What is a 13-week cash flow forecast?

A 13-week cash flow forecast projects a business's cash balance week by week for the next quarter. Each week starts with the prior week's ending cash, adds expected receipts, subtracts expected disbursements, and arrives at a new ending balance. Thirteen weeks is long enough to see a problem coming and short enough to forecast from known invoices, payroll dates and bills.

It is a direct cash forecast — built from cash in and cash out — rather than a projection derived from the income statement. That is what makes it useful in tight-cash situations: it shows the specific week the balance dips, not just the quarter's average.

The formula

Ending cash (week n) = ending cash (week n − 1) + receipts (week n) − disbursements (week n)

• This starter uses one average weekly inflow and outflow, so the balance moves in a straight line: ending cash at week 13 = starting cash + 13 × (weekly cash in − weekly cash out).
• A full forecast replaces the averages with line items by week: customer receipts by expected payment date, payroll on pay dates, rent, debt payments, tax deposits, and one-off items.
• Each week, roll forward: replace the oldest forecast week with actuals and add a new week 13.

Worked example

Illustrative example — a hypothetical contractor starting the quarter with modest cash:

1. Starting cash: $260,000. Average weekly cash in: $118,000. Average weekly cash out: $126,500.
2. Net weekly change = $118,000 − $126,500 = −$8,500.
3. Week 13 ending cash = $260,000 + 13 × (−$8,500) = $260,000 − $110,500 = $149,500.
4. Add a one-off $45,000 payment in week 9 that isn't in the averages: week 9 drops from $183,500 to $138,500, and week 13 ends at $104,500.

The averages say the business loses $8,500 a week and stays comfortably positive. One unmodelled payment takes $45,000 off every week that follows. That is why a real 13-week forecast is built line by line.

How to read your result

The lowest weekly balance is the number to watch, not week 13. If the trough comes close to zero — or to the minimum balance your lender requires — that is the week to plan around.

A steady negative weekly net change is a burn rate. Divide starting cash by it to see how many weeks you have; for a monthly view, use the cash runway calculator.

Compare forecast to actual every week. Persistent misses in the same direction (receipts always later than expected, for example) tell you which assumptions to fix.

Common mistakes

• Forecasting from the income statement. Revenue is not receipts and expenses are not payments. Use expected collection dates and payment dates.
• Smoothing lumpy items. Payroll, rent, debt service and tax deposits land in specific weeks. Averaging them hides the trough.
• Never rolling it forward. A forecast built once and left alone goes stale within a few weeks. Roll it weekly and compare it to actuals.
• Optimistic receipts. Forecast collections from how customers actually pay, not from invoice due dates. Your DSO tells you the gap — see the DSO calculator.

Frequently asked questions

Further reading

• Killing the annual budget: why rolling forecasts win — The 13-week forecast is the short end of a rolling forecast.
• Upskilling your finance team for the AI era — Building and rolling a cash forecast is one of the skills worth developing in-house.
• Revenue recognition without the headache — Why booked revenue and cash receipts drift apart — and what that does to your forecast.
• Cash flow forecasting is broken. Here's how to rebuild it. — Going from a spreadsheet to a forecast you trust.

More free tools

• Cash Runway Calculator
• Loan Covenant Health Check
• DSO & AR Aging Calculator
• Gross Margin Analyzer
• DSCR Calculator
• Covenant Headroom Checker

Worked examples on this page are illustrative and use hypothetical figures, not customer data. This page is general information, not lending, tax or investment advice.