How to Build a 13-Week Cash Flow Forecast Without a Finance Team
Most small businesses find out they have a cash problem in the week it arrives. Payroll is on Friday, the account has ₦900,000 in it, and the ₦2,400,000 invoice you were counting on is not going to land in time. By then your options are all bad: borrow at a terrible rate, delay salaries, or beg a supplier for two more weeks.
A 13-week cash flow forecast is the cheapest way to see that Friday coming while you can still do something about it. It is a spreadsheet, nothing more. You can build the first version in an afternoon and update it in fifteen minutes every Monday. You do not need an accountant, and you do not need to be profitable for it to matter, because being profitable and missing payroll is a normal way for good businesses to go broke.
Why 13 weeks
Thirteen weeks is one quarter, and a quarter is the horizon where you can still act. Look at a shorter window and you only see problems you can no longer fix. Look much further out and the numbers turn into guesses.
Thirteen weeks is also long enough to cover a full collection cycle. A Net 30 invoice that goes 30 days late still lands inside the window, so the forecast shows you what your current receivables will really turn into.
The weekly part matters as much as the 13. A monthly forecast can look comfortable while you are broke on the second Friday, because payroll and a supplier payment both land before your customers pay. Cash problems are timing problems, and months hide timing.
Open with real cash, then forecast collections
Line one is what is in the bank on Monday morning. Not what your books say you are worth, not what you are owed: what you could spend today. If the money is spread across accounts, use the total you can actually move. That number is the only fact in the sheet. Everything under it is an estimate, and the job is to keep those estimates honest.
Under it, place every unpaid invoice in the week you expect the money to arrive, not the week it is due. Those are different weeks, and the whole forecast turns on the difference.
Take the due date, then adjust for how that customer actually behaves. If a distributor is on Net 30 and has paid you an average of 18 days late for the past year, the ₦1,200,000 they owe belongs in week 7, not week 4. Your aging report is where this evidence lives, and if you do not keep one yet, start there.
Be blunt about invoices already deep in arrears. An invoice 90 days overdue from a customer who has stopped answering the phone is not week 3 income. Leave it out and treat it as a collections problem instead. A forecast you have flattered is worse than none, because you will trust it.
Then add anything else real: a deposit on a signed job, a retainer, a refund you have confirmed. Only money you can name and date.
Lay out the money going out
Under collections, list every outflow in the week it hits. Payroll on its actual dates. Suppliers on their due dates. Rent, diesel, internet, subscriptions, loan repayments. Tax remittances, which land on known dates and are the ones people forget until the week they are due.
Do not average things out or smooth them. If ₦3,000,000 of supplier payments all fall in week 5, then that is what week 5 looks like. The lumpiness is the information you came for.
Read the bottom row
Each week: opening cash, plus collections, minus outflows, equals closing cash. Closing cash becomes next week's opening cash. That is the entire model, and there is no step you are missing.
Now run your eye along the bottom row and find the first week that goes negative or gets uncomfortably close to zero. That week is the reason you built the sheet.
Say week 6 closes at negative ₦450,000. You are seeing it in week 1, so you have five weeks to fix a ₦450,000 hole, and the fixes are ordinary ones. Pull the ₦1,200,000 invoice in week 7 forward to week 5. Ask one supplier to move a payment by ten days. Invoice the job you finished last week instead of waiting for month end. Any one of those closes the gap, and none of them are available to you on the Friday itself.
Update it every Monday. Roll the window forward a week and correct last week's guesses against what actually happened. After a month or two your collection estimates get noticeably sharper, and you learn which customers you consistently over-forecast.
Where the hard numbers come from
The difficult half of a 13-week forecast is not the arithmetic, it is knowing when each invoice will really pay. Arvalox keeps that half current: it tracks every outstanding invoice, ages it by risk, and shows you which customers run late and by how much, so the collections side of your sheet reflects what your customers actually do rather than what their terms say. Payroll, rent, and suppliers you already know by heart.