How to Build a 13-Week Cash Flow Forecast

Why Thirteen Weeks

A P&L tells you if you're profitable. A 13-week cash flow forecast tells you if you can make payroll in six weeks. These are different questions, and more businesses die from failing the second than the first.

Thirteen weeks is the sweet spot: one full quarter, long enough to see around corners, short enough that your assumptions aren't fiction. It's the standard tool in turnaround work for a reason, but you shouldn't wait for a crisis to build one.

The Structure

The forecast is a grid: weeks across the top, cash categories down the side. Each week shows cash in, cash out, and the running balance.

Starting point: your actual current cash balance (all operating accounts, today).

Cash in, by week:

• Customer collections (based on your AR aging and real payment behavior, not invoice dates)
• Recurring revenue you can count (retainers, subscriptions)
• Everything else: loans, owner injections, asset sales, tax refunds

Cash out, by week:

• Payroll and payroll taxes (the fixed anchor: it lands when it lands)
• Rent and other fixed obligations
• AP payments (scheduled by actual due dates, not averaged)
• Debt service
• Quarterly tax estimates
• One-time items you know are coming: annual insurance, software renewals, bonuses

The bottom line: each week's ending cash becomes next week's beginning cash. The running balance is the whole point. It's the number that answers "when do we dip below comfortable?"

Building It in an Afternoon

Step 1: Start from your AR aging

Export your open invoices. For each one, assign the week you actually expect payment, based on that customer's history, not the due date. Your reliable payer who always runs net-45 goes in week 6, not week 4.

Step 2: Schedule your AP the same way

Every open bill, placed in the week you'll actually pay it. Then layer in the recurring outflows that aren't bills: payroll dates, rent, loan payments, subscriptions.

Step 3: Add the known lumpy items

Scroll your calendar and last year's bank statements for the non-monthly hits: quarterly estimated taxes, annual premiums, annual software contracts, planned equipment purchases. These are what blow up naive forecasts.

Step 4: Fill in recurring revenue conservatively

Only the revenue you'd bet on: signed contracts, active subscriptions, retainers. Pipeline does not belong in a 13-week forecast unless it's nearly closed and the customer pays fast.

Step 5: Compute the running balance and stare at it

Find the lowest week. That number (and how far out it sits) is the health of your next quarter in one figure.

Running on It Weekly

The forecast is a living tool, not a quarterly artifact:

• Every Monday (or Friday): roll it forward. Drop the week that passed, add week 14, update actuals.
• Update expected collections the moment a customer's behavior changes.
• When a gap appears, the forecast tells you how big it is and how many weeks you have to act: accelerate collections, delay discretionary spend, draw on the credit line before you're desperate, which is when banks say yes.

The Discipline That Makes It Work

Base collections on behavior, not terms. Hope is not a forecasting method.

Separate committed from discretionary outflows. When the low week comes, you need to know instantly what can move.

Keep it honest. A forecast tuned to tell you what you want to hear is worse than none. It manufactures false comfort.

Common Mistakes

Forecasting revenue instead of collections. The sale isn't cash until it clears.

Averaging expenses. Cash leaves on specific dates. "About $40K a month" hides the week where payroll, rent, and the quarterly tax estimate collide.

Building it once. An un-rolled forecast is expired within a fortnight.

Get the Template

We've built a ready-to-use 13-week cash flow template with the structure above pre-wired: weekly columns, running balance, committed vs discretionary split. And if you'd rather have this maintained for you, a rolling 13-week forecast is part of every fractional CFO engagement we run. Either way: build it before you need it.