The weekly hour that ends the overdraft surprise. Most small businesses discover their cash position the way you discover a low tyre: at the worst possible moment, and only once it has already cost something. The P&L says the business is fine; the bank says Tuesday is a problem; and the gap between the two is timing — invoices that age while payroll and the BAS sit on fixed dates.
The fix is one spreadsheet tab, four rows, thirteen Mondays, and one hour a week. Here is the whole system.
Why thirteen weeks — not a month, not a year
- A month is too short. It hides the week inside the month where payroll, rent and the tax instalment land together. Businesses don't run out of money "in March"; they run out on the Thursday of week 5.
- A year is too vague. An annual budget is a set of opinions. At thirteen weeks the inputs are named invoices and dated runs — commitments, not hopes.
- Thirteen weeks is a full quarter. It catches every recurring beast at least once: rent, payroll, BAS, super, supplier end-of-month terms, the seasonal dip you always forget.
- It is the length of the future you can still change. A hole thirteen weeks out has eleven solutions. A hole on Friday has two, and both are phone calls you didn't want to make.
The four rows — the whole instrument
- Cash in. Only money that clears. Collections by customer, moved to the week you believe based on real behaviour — not the due date printed on the invoice.
- Cash out. The five fat lines each get their own row: payroll, rent, suppliers (by terms), tax and super (dated, immovable), and one thin long-tail line for everything else.
- Opening balance. Last week's actual closing balance, from the bank's word — never from the forecast.
- Closing balance. Opening + in − out. Colour-coded against a floor you agreed once — commonly four weeks of payroll — not re-litigated on a frightened Friday.
The Monday, nine o'clock ritual
Pull last week's actuals (5 min) → roll the window one week forward (2 min) → update every invoice promise to the week it will really clear (10 min, the highest-value ten minutes of the hour) → scan the thirteen closing Mondays and decide on any amber or red week today → write one line about what moved and what you're watching.
Same hour, same chair, one owner. A forecast that updates "when there's time" is a spreadsheet of the dead; the ritual is the instrument.
The three rules that keep it honest
- Book the cheque, not the promise. "He always pays around the 15th" goes in on the 15th only if it has cleared there three months running. Collection optimism is the #1 corruption of small-business forecasts.
- The tax calendar is rows, not weather. BAS, super and payroll tax are the most predictable numbers in the business — every quarter, same shape. If they surprise you, they were never rows.
- The floor is agreed once, in writing. Four weeks of cover, or whatever your nerves require — decided on a calm Monday, not negotiated on a frightened one.
Worked example — the joinery contractor's week 5
A nine-person joinery contractor, $1.9M revenue, profitably busy for years — and every quarter there was still a fortnight where the owner checked the account before payroll ran. One wet Monday he built the sheet: two hours, four rows. Week 5 closed red: a $68,400 hole — three progress claims, payroll and the BAS instalment all landing on the same days.
Because it was week 5 and not Friday, there were eleven options: two claims invoiced same-day instead of "when the site wraps", the timber order moved onto 30-day terms agreed in one phone call, and a discretionary $9,800 hire moved three weeks right. Week 5 closed at +$11,200. Payroll never checked the weather again; the overdraft was cancelled at renewal because it hadn't been touched. His line: "The hole was always there. The only thing that changed is that I met it eleven weeks early, with options, instead of on the day, with none."
Free tools for this
The full page — with the four-row template spec, the floor rule, and the five traps (profit-vs-cash, tax-as-weather, collection optimism, the sheet built once and never rolled, three owners and no owner) — is free on the site:
👉 The 13-Week Cash Flow Forecast — full playbook
If you want the paid versions of this thinking:
- The First 30 Minutes — free incident quick-start checklist
- Ops Starter Kit — incident response for small teams — $14
- Ops Starter Kit Vol. 2 — advanced incident response & communications — $27
- Ops Mega Bundle — all 5 kits in one download — $49
Related reading: the debtor days monthly review is the truth about how fast receivables really turn into cash; the cash runway checklist is the survival instrument this forecast keeps you from ever needing; the supplier payment terms checklist is the lever on the outflow side.
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