The 13-week cashflow forecast, explained without the jargon

Why 13 weeks is the sweet spot for small business cashflow planning, what goes in each row, and how to build your first one in an afternoon.

Most small businesses find out they have a cash problem about two weeks before it becomes a crisis. The invoice that was “definitely going out Friday” slips. A VAT bill lands. Payroll is Thursday. Suddenly you’re refreshing your banking app instead of running your business.

A 13-week cashflow forecast is the cheapest insurance against that. It isn’t accounting, it isn’t a budget, and it doesn’t need to be perfect to be useful.

Why 13 weeks?

Thirteen weeks is one quarter. It’s long enough that you can actually do something about what you see — chase a debtor, delay a hire, draw on a facility — and short enough that you can forecast week by week without inventing numbers.

Monthly forecasts hide the problem. A month can show a healthy net position while you were technically unable to make payroll in week two. Cash is a daily reality, so weekly is the coarsest view that still tells the truth.

What actually goes in it

Four blocks, in this order:

  1. Opening balance — what’s genuinely in the bank on Monday morning.
  2. Cash in — customer receipts, by expected payment date, not invoice date. This is the row people get wrong most often.
  3. Cash out — payroll, rent, suppliers, loan repayments, tax. Put the tax dates in now; they’re the ones that ambush people.
  4. Closing balance — opening + in − out. This becomes next week’s opening.

That’s it. If you want a fifth row, make it your minimum acceptable balance, so a problem week is visually obvious rather than something you have to calculate.

Invoice date is not payment date

If your terms are 30 days and your customers typically pay at 45, forecast 45. Use what actually happened over the last six months, not what the contract says. Optimism in this row is the single biggest reason forecasts go wrong.

A quick way to get honest: for each regular customer, look at the last five invoices and take the median days-to-pay. Use that number.

Update it weekly, not monthly

The value isn’t in the forecast — it’s in the variance. Every Monday, replace last week’s estimate with what actually happened and ask one question: where was I wrong, and why?

After about six weeks, your forecast stops being guesswork and starts being a model of how your business actually behaves. That’s the point where it starts earning its keep.

Wrapping up

Build the first one in a spreadsheet this week. Thirteen columns, four rows, one hour. If it shows you a bad week in week nine, you’ve just bought yourself two months to fix it.

When the spreadsheet starts creaking — and it will, around the time you add scenarios — Promeflow keeps the same model updated from your actual numbers.