Cash flow forecasting: the 13-week model.

A cash flow forecast projects where your cash balance will be in the weeks ahead — not where it was last month. The most useful version for most businesses is the 13-week rolling forecast: a week-by-week projection of cash in and cash out over the coming quarter, updated every week. It’s the difference between seeing a cash crunch coming in time to act and discovering it when a payment bounces.

Profit and cash are not the same thing. A business can be profitable on paper and still run out of money — growth ties up cash in receivables and inventory, timing mismatches bite, and the bank balance tells you nothing about what’s due next week. A forecast is how you manage the cash reality underneath the profit.

What a 13-week cash flow forecast is.

It’s a rolling, week-by-week projection covering roughly a quarter ahead. Each week shows expected cash inflows (customer collections, other receipts), expected cash outflows (payroll, vendors, rent, debt, taxes), and the resulting projected cash balance. Because it’s rolling, each week you drop the week that just passed and add a new week at the far end, so you’re always looking a full quarter forward.

Why 13 weeks?

Thirteen weeks is one quarter — long enough to see meaningful problems coming (a seasonal dip, a big tax payment, a project that won’t collect for two months) while still being short enough to forecast with real accuracy. Beyond a quarter, weekly cash timing becomes guesswork; inside a quarter, you can project it week by week with confidence. It’s the sweet spot between visibility and reliability, which is why it’s the standard tool for hands-on cash management.

How to build one.

1. Start with today’s actual cash balance. The forecast is only as good as its starting point, so begin from real, reconciled cash.
2. Lay out 13 weeks across. One column per week for the coming quarter.
3. Project cash inflows by week. Based on your AR aging and expected collection timing — when will invoices actually be paid, not when they were issued — plus any other receipts.
4. Project cash outflows by week. Payroll on its dates, vendor payments by terms, rent, loan payments, and — critically — the lumpy items people forget: quarterly taxes, insurance, annual renewals.
5. Calculate the running balance. Each week’s starting cash, plus inflows, minus outflows, gives the ending balance that carries into the next week. Now you can see any week the balance runs dangerously low.

How to use it every week.

The forecast earns its value in the weekly rhythm. Each week, update it with what actually happened, roll it forward one week, and compare projection to reality — the gaps teach you to forecast better and reveal problems early. Most importantly, act on what it shows: if week nine runs tight, you have eight weeks to accelerate collections, delay a discretionary payment, or arrange financing — calmly, instead of in a crisis. Businesses running on EOS often put the 13-week forecast right into their weekly leadership meeting for exactly this reason.

Why the forecast depends on clean books.

A forecast is only as reliable as the data beneath it. If your AR aging is wrong, your inflow projections are wrong; if expenses aren’t recorded properly, your outflows are guesswork. That’s why cash forecasting sits on top of reliable bookkeeping and a disciplined month-end close — the forecast is the forward-looking layer, and it needs an accurate backward-looking foundation. Building and running that forecast, and turning it into decisions, is core fractional CFO work.