How to Build a 13-Week Cash Flow Forecast

A 13-week cash flow forecast shows what your bank balance will be each week for the next quarter. Here is how to build one, with a worked example, and how to use it.

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5 min read

Published On

Oct 30, 2026

Read: 6 minutes

Last updated: November 6, 2026

In this guide

What a 13-week forecast is
How to build one in five steps
A worked example
How to use it every week

What Is a 13-Week Cash Flow Forecast?

A 13-week cash flow forecast is a week-by-week view of the cash coming into your business, the cash going out, and what your bank balance will be at the end of each week.

Thirteen weeks is one quarter. That is long enough to see a problem coming and short enough to estimate with real numbers. Monthly budgets hide the detail that matters, because a business can look fine for the month and still run short on the Friday payroll lands before a big customer pays.

Your accounting software tells you what already happened. A forecast tells you what is about to happen, while you can still do something about it.

How to Build One in Five Steps

You need a spreadsheet and about an hour. Put the 13 weeks across the top and your cash lines down the side.

Step 1: Start with today's real balance

Use the number in the bank today, not the number in your books. Everything else in the forecast builds on this figure, so it has to be right.

Step 2: List the money coming in

Go through what you expect to collect, week by week:

Invoices already sent, placed in the week you expect to be paid, not the week they are due
Regular sales, based on the same weeks last year or your recent average
Anything else, like a deposit on a signed job

Look at how your customers actually behave. If a customer on 30-day terms always pays in 45 days, put that money in week seven.

Step 3: List the money going out

Fixed costs are the easy part: payroll, rent, insurance, software and any debt payments. Put each one in the exact week it leaves the account.

Then add the variable costs, like supplier orders, materials and shipping. Finish with the lumpy items that are easy to forget, like annual renewals, tax remittances and a repair you already know is coming.

Step 4: Work out each week's closing balance

For each week, take the opening balance, add the money in and subtract the money out. That closing balance becomes the next week's opening balance.

Closing balance = opening balance + money in − money out

Step 5: Mark your minimum balance

Pick the smallest balance you are comfortable with, such as two weeks of payroll. Highlight any week that falls under it. Those are the weeks that need a decision.

A Worked Example

Say a business has $40,000 in the bank and wants to stay above $30,000. It collects $20,000 a week. Regular costs are $10,000 a week, and payroll of $15,000 goes out every second week. A $12,000 supplier order is due in week four. A customer who owes $12,000 is due to pay that week, but always pays two weeks late, so only $8,000 will come in.

Week 1: $40,000 + $20,000 − $10,000 = $50,000
Week 2: $50,000 + $20,000 − $25,000 = $45,000
Week 3: $45,000 + $20,000 − $10,000 = $55,000
Week 4: $55,000 + $8,000 − $37,000 = $26,000

Put that invoice on its due date and week four closes at $38,000, safely above the minimum. Put it on the date this customer really pays and week four closes at $26,000, below it. Same business, same sales, and three weeks of warning that the first version never gives you. That warning is time to chase the customer, move the supplier's date, or line up funding.

The Slate View

Most forecasts predict sales. If you invoice, the better ones predict people. The number that moves your balance is often not how much you will sell, but when each customer will really pay, and your existing customers have already shown you.

The tightest week is also the one that counts. From the other side of the table, a strong month does not cancel a week the balance nearly hit the floor. Find that week yourself while your recent months still look good, because that is when you have the most choices.

How to Use It Every Week

A forecast is only useful if it stays current. Pick one time each week, and spend 15 minutes on it:

Replace last week's estimates with what actually happened.
Add a new week at the end, so you always see 13 weeks ahead.
Check where your estimates were wrong and why.
Look for any week under your minimum, and decide what to do now.

After a month or two, your estimates will get noticeably better, because you will learn how your customers and costs really behave.

What to Do When You See a Gap

Start with what costs nothing. Collect sooner, move a purchase, or agree new payment dates with a supplier. If a gap remains, your working capital shows how much cushion you really have, and your cash runway shows how long the current pace can last.

When outside funding makes sense, arrange it before the tight week arrives. See how a credit line compares with a cash advance.

You can also ask Slate Signals in Claude or ChatGPT how your business is really doing, using your own numbers.