Guide

How to forecast cash flow when clients pay 30 to 90 days later

Runway · September 2026 · General information, not financial advice

If you do the work first and get paid later, the week that hurts is rarely a slow week. It's usually a busy one: materials, payroll and subcontractors go out now, and the money for that work arrives 30 to 90 days after you finish.

A monthly profit report can show a good month while your bank balance is on its way below zero. A weekly cash forecast shows you that week ahead of time, while there's still time to do something about it.

This guide walks through the method step by step. If you'd rather start from a working spreadsheet, the free template does all of this for you.

1. Forecast by the week, not the month

Payroll goes out every week or every two weeks. A monthly view averages everything together, so the week where payroll lands before a big payment comes in simply disappears.

Look 12 to 13 weeks ahead. Many finance teams use a 13-week cash forecast for the same reason: it's far enough out to see a gap coming and act on it, and close enough that the numbers are still realistic. Further out, the numbers get less reliable.

2. Plan for when money actually lands

The invoice date isn't when you get paid. For each invoice you're waiting on, estimate the paid date:

Expected paid date = invoice date + payment terms + how late this client usually pays

Example: an invoice sent on March 1 on net-30 terms, to a client who usually pays about 10 days late, should be planned for around April 10, not March 31.

For work you haven't invoiced yet, use the date you'll send the invoice.
For clients who always pay late, write down how late. That one number fixes a lot of forecasts.
Leave overdue invoices out until the client gives you a new date. Be cautious about money coming in, and realistic about money going out.

3. Put every payment on the date it leaves

List what goes out in three groups:

Every week: payroll, fuel, small tools.
Every month: rent, loan and lease payments, phones and software, on the day they're actually paid.
One-off: materials, subcontractors, equipment rental, each on its due date.

Watch the one-off group closely. Materials for a big job are usually paid long before that job pays you.

4. Set money aside for lumpy costs every week

Quarterly estimated taxes, insurance renewals, annual licenses and truck repairs don't come often, which is exactly why they wreck a plan. Instead of letting them land all at once, set a little aside each week:

Weekly set-aside = (amount − what you've already saved) ÷ weeks until it's due

Example: a $9,000 quarterly tax payment due in 7 weeks, with $3,000 already saved, means setting aside about $857 a week.

Then treat that money as spoken for. What's actually yours is your bank balance minus what you've set aside.

5. Find your lowest point, then your safe-to-spend

Run the weeks forward: each week starts from last week's balance, adds money in, and subtracts money out and set-asides. Then look for the lowest week in the next 12.

Safe to spend now = your lowest point in the next 12 weeks − your safety buffer

Why the lowest point? Anything you spend today lowers every week after it by the same amount.

Example: if your lowest point is $8,000 in week 6 and you never want to go below $5,000, you can spend about $3,000 today, even if the bank shows $30,000.

6. When you see a short week coming

The point of the forecast is to see the problem while you still have options. Some common moves (check big decisions with your accountant):

Invoice as soon as the work, or a stage of it, is done, instead of at month end.
Ask for deposits or progress payments on larger jobs.
Follow up with slow payers before the due date, not after.
Time large material orders, or ask suppliers about payment terms.
Arrange a line of credit before you need it. It's easier to set up when you're not in a crunch.
If quarterly taxes keep causing the gap, ask your accountant whether your estimates are set right.

7. A Monday routine

A forecast only helps if it stays current. Once a week:

Update your bank balance.
Mark invoices that got paid, and add new ones.
Add any new one-off payments.
Check your lowest point and your safe-to-spend.

Then the window moves forward a week, and you do it again next Monday.

Get the free template

Our free 12-week template does everything in this guide: expected paid dates, weekly set-asides for lumpy costs, and safe-to-spend. It comes filled with an example business so you can see it work. Excel and Google Sheets, no signup.

Copy to Google Sheets Download for Excel See the template page

Running several jobs at once? The Pro version ($17) dates every deposit, progress and final payment for you.

Rather not keep a spreadsheet? Try the free web version of Runway: the same forecast in your browser, no signup.