Building and steering a 13-week cash flow forecast

Quick answer. A 13-week cash flow forecast is a table of 13 weekly columns (one quarter) that you build in six steps and update every Monday. It is built on a simple spreadsheet in two to three hours, then maintained in a 30-minute weekly review. Its value is not perfect precision: it is the decision cadence. It reveals a cash gap two to three weeks before it appears on the bank statement.

This hands-on manual explains how to build the table from scratch and keep it alive. For the theoretical framework (why 13 weeks, when to activate it, sector adaptation, stress test), see the complete 13-week cash plan methodology. For fundamentals, see our cash management methodology.

Thirteen weeks represents roughly a quarter: long enough to anticipate major deadlines (VAT, payroll taxes, rent, payroll, corporate tax instalments), short enough to stay precise and actionable. The format, born in finance functions and turnaround funds, is now within reach of any small business on a spreadsheet.

It is aimed first at founders under cash tension, in periods of commercial uncertainty, or in a growth phase that consumes cash. With comfortable, stable cash, a 12-month monthly plan may suffice; the 13-week format then complements, rather than replaces, it.

The build comes down to six steps chained over half a day:

1. Set the opening bank balance and create the 13 weekly columns.
2. Place receipts in their actual cash week, based on the DSO.
3. List disbursements (payroll, taxes, suppliers) by their real due dates and DPO.
4. Compute, for each week, the net variation, the end-of-week balance and the gap to the alert reserve.
5. Feed the table every Monday by freezing actuals and adding a week.
6. Hold a 30-minute weekly review to decide.

Steps 2 and 3 rest on a key notion: the gap between invoice date and cash date. That is the whole point of the cash conversion cycle (DSO, DPO, DIO).

Steps 1 and 2: the opening balance and receipts (DSO)#

Start with the real bank balance on the Monday of week W, then structure the lines into simple blocks. The golden rule: each flow goes in the week the money actually moves, not the invoice week.

So an invoice issued in week W1 with a six-week collection period (DSO) sits in the receipts of W7, not W1. This granularity exposes peaks invisible in a monthly plan.

Steps 3 and 4: disbursements (DPO) and the end-of-week balance#

Then list all disbursements in their real due week: net salaries, payroll taxes, VAT, rent, subscriptions, loan instalments, and supplier payments per the delay you actually practise (DPO). Do not forget tax deadlines set by the legal calendar: corporate tax instalments on 15 March, 15 June, 15 September and 15 December, monthly or quarterly VAT depending on your regime.

For each week, compute the net variation (receipts minus disbursements), then the end-of-week balance. Compare that balance to a minimum operating reserve — typically enough to cover the next payroll, the quarter's payroll taxes and one month of rent. If the projected balance drops below this reserve, the spreadsheet must flag it (red conditional formatting). This is the heart of the alert mechanism.

The 13-week forecast is only worth its rolling logic. Every Monday (or Tuesday morning), three actions suffice: freeze the past week with actual figures, add a new week to the horizon to always keep 13 weeks ahead, and measure the forecast-versus-actual gap.

That gap is precious. If, week after week, the forecast overestimates receipts by 5 to 10%, it is not a detail: it signals a lengthening DSO or commercial deterioration. A plan updated monthly loses most of this early-warning value.

The weekly review focuses on a few indicators, not the detail of every line.

Always distinguish real cash from merely available lines (authorised overdraft, Dailly assignment, factoring): in tense periods, a theoretical line can be suspended without notice.

Take a services agency of 8 people, opening balance €45,000, 45-day DSO. While building the table, the founder sees that week W6 concentrates three disbursements: payroll, the 15 June corporate tax instalment (due only where the prior year's corporate tax exceeds €3,000) and a large supplier payment. Against them, an important customer's receipt will only arrive in W8 due to the DSO. The projected W6 balance falls to €6,000, below the €30,000 reserve.

Spotted in W2, this gap leaves time to act: targeted chasing of the customer to bring the receipt forward, a one-month deferral request to the accountant for a supplier payment, and preparation of a line drawdown as a safety net. In W6, the balance stays above the reserve and the line is not drawn. Without the table, the gap would only have shown on the bank statement, too late to negotiate.

For a startup without stabilised revenue, the format reduces to disbursements (payroll, hosting, services) and inflows (raise, R&D tax credit, JEI status). It mainly tracks the runway, i.e. the number of weeks of cash remaining. See our chartered accountant sector page for startups and our outsourced CFO for startups and SMEs.

For a seasonal activity, the 13-week plan should cover at least one season cycle: it then helps anticipate the trough after the peak, when costs continue but receipts slow. In very uncertain periods, pair the central scenario with a cash stress test.

• Confusing invoice date with cash date: VAT and DSO create gaps that distort the whole forecast.
• Updating monthly: the format loses most of its value without the weekly rolling.
• Forgetting the operating reserve: a plan that only alerts at zero alerts too late.
• Systematic commercial optimism: overestimating receipts creates an invisible breaking point.
• Ignoring the 2026 calendar: the e-invoicing rollout (mandatory receipt from 1 September 2026) reshapes invoice rhythm, and the end of the simplified VAT regime on 1 January 2027 changes filing deadlines.

In our practice, the difficulty is almost never technical: the table is built in a few hours. The difficulty is weekly discipline and a decision culture. A founder who spends 30 minutes each Monday arbitrating on the basis of the table crosses a threshold no tool alone provides. For structures without a finance function, our chartered accounting firm in Paris 8th often acts as this trusted third party.

One effect is regularly underestimated: the banking relationship. An SME presenting its bank with a held and up-to-date 13-week forecast changes category in the relationship manager's informal assessment. It is a concrete relational asset that translates into financing conditions. The risk is abandonment when tension eases: keep the format in a lighter version during calm periods rather than rebuilding everything at the next crisis.

Hayot Expertise tip. Start with the spreadsheet, not the tool. Validate the method and the Monday cadence over three or four weeks first; you will migrate to a steering tool such as Finthesis once the format stabilises. Before any short-term financing, optimise working capital first: see our 9 levers to free cash.