Why 13 Weeks Is the Right Horizon
The 13-week (one-quarter) forecast horizon is the result of a practical optimisation: it is long enough to see structural problems before they become crises, and short enough that the individual week-level estimates are grounded in known inputs rather than speculative assumptions.
A four-week model does not provide sufficient early warning. If a cash trough will occur in week 10, a four-week model will not show it until you are already four weeks away from it. At that point, the options for remediation are severely constrained: payroll cannot be renegotiated on four weeks' notice, major supplier payments are already committed, and an emergency funding raise requires more lead time than is available. Four weeks is too little.
A 26-week model is too uncertain to be actionable in the outer weeks. Revenue forecasts beyond 13 weeks begin to carry substantial uncertainty for most growth-stage businesses, and presenting a 26-week model with precision-formatted numbers in weeks 14 to 26 gives a false impression of accuracy. The board should understand that the outer half of a 26-week model is closer to a scenario illustration than a forecast. This distinction is better served by a separate scenario model for strategic planning purposes.
The 13-week horizon is also aligned with the quarterly reporting cycle used by most institutional investors. Presenting the cash trough and minimum headroom for the current quarter at each board meeting creates a consistent and comparable picture over time. Board members who see the same format with updated actuals at each quarterly meeting quickly develop an intuitive calibration for what "good" and "concerning" look like on your specific business.
The Receipts and Payments Method
The 13-week forecast must use the receipts and payments method, not the indirect method. This distinction is critical and is not always well understood in finance teams.
The indirect method starts with profit or loss and adjusts for non-cash items and working capital movements. It is used in the statutory statement of cash flows and is appropriate for historical reporting. It is not appropriate for short-term cash forecasting because it requires a complete income statement and balance sheet to produce an output that is less directly tied to the actual timing of cash movements than necessary for operational purposes.
The receipts and payments method forecasts actual cash movements in the bank account, week by week. Receipts are cash in; payments are cash out. The model does not care about accruals, provisions or non-cash items. The closing cash balance each week is the opening balance plus receipts less payments. Every line in the model should correspond to a payment that will appear on the bank statement.
How to Build the Model
The model has a simple structure: columns are weeks (weeks 1 to 13, with a pre-filled actuals column for the current week), and rows are categories of receipts and payments. Here is a template structure showing the key row categories and column headers.
| Row Category | W1 (Actual) | W2 | W3 | W4 | W5–W8 | W9–W13 |
|---|---|---|---|---|---|---|
| Opening Balance | ||||||
| Opening cash balance | Actual | = prior close | — | — | — | — |
| Receipts | ||||||
| Customer receipts (B2B invoices) | Actual | DSO-based | — | — | — | — |
| Customer receipts (subscriptions / DD) | Actual | Fixed dates | — | — | — | — |
| VAT reclaims | — | Quarterly | — | — | — | — |
| Other receipts (grants, R&D credit) | — | Known dates | — | — | — | — |
| Total Receipts | SUM | SUM | — | — | — | — |
| Payments | ||||||
| Payroll (salaries + PAYE/NIC) | Fixed dates | — | — | — | — | — |
| Supplier payments (COGS) | DPO-based | — | — | — | — | — |
| Supplier payments (opex) | DPO-based | — | — | — | — | — |
| VAT payment (quarterly) | — | Quarter-end | — | — | — | — |
| Corporation tax instalments | — | Known dates | — | — | — | — |
| Loan / debt repayments | Fixed dates | — | — | — | — | — |
| Capital expenditure | Known dates | — | — | — | — | — |
| Total Payments | SUM | SUM | — | — | — | — |
| Net cash movement | R - P | R - P | — | — | — | — |
| Closing cash balance | Actual | Forecast | — | — | — | — |
| Metrics | ||||||
| Minimum liquidity threshold | Input | Input | — | — | — | — |
| Cash headroom above threshold | Closing - threshold | — | — | — | — | — |
Key Inputs and How to Estimate Them
The quality of the 13-week forecast depends entirely on the quality of the inputs. The receipts and payments method requires you to estimate the actual timing of cash movements, not the accruals period in which they are recognised. This is a different skill from financial modelling, and it is the skill that most finance teams take six to nine months of actual operation to develop properly.
Customer Receipts
For B2B invoicing, customer receipts are estimated by applying the average days sales outstanding (DSO) to the weekly invoicing run. If your current DSO is 42 days (six weeks), an invoice raised in week 1 is expected to be collected in week 7. In practice, DSO varies by customer: large enterprise customers often pay on 60-day terms; SME customers may pay in 30 days. A more granular approach maintains a collection schedule at the individual invoice level for large invoices and uses an aggregate DSO estimate for the long tail of smaller receivables.
For subscription businesses, the receipt pattern depends on the billing cycle. Direct debit collections run on fixed dates (typically the 1st or 15th of the month). Annual upfront payments arrive in specific weeks based on the customer's subscription anniversary date. The model should have a direct link to the billing system export wherever possible.
Payroll
Payroll is the most predictable line in the model. Salaries are paid on a fixed date each month (or last working day of the month). Employer PAYE and National Insurance contributions are paid to HMRC on the 22nd of the following month (for businesses paying electronically). Pension auto-enrolment contributions are typically paid within a defined number of days of payroll processing. All of these dates can be pinpointed precisely for the 13-week horizon.
Supplier Payments
Supplier payments are estimated by applying your days payable outstanding (DPO) to the weekly purchase order or invoice run. If your standard payment terms are 30 days net, a supplier invoice approved in week 1 is expected to be paid in week 5. For businesses running a weekly payment run, the timing is further constrained to specific days of the week.
"The outer weeks of a 13-week forecast will always be less precise than the inner weeks. This is expected and appropriate: the value of the model is not precision in week 13, but the early identification of structural cash troughs that require action now. Waiting for certainty before acting on a cash forecast is the most dangerous approach possible."
How to Keep the Model Current
A 13-week model that is not updated weekly is not a management tool; it is a historical document. The weekly update process should take no more than two hours for an experienced finance team member, and the steps are consistent week by week.
- Roll the model forward by one week: delete week 1 (now past), add a new week 13, and populate actuals for the week just ended from the bank statement.
- Compare actuals to the prior week's forecast for the week just ended: for each material line, identify the variance and the reason. Is a customer collecting later than expected? Has a supplier payment been brought forward? This variance analysis is the most valuable part of the process.
- Update the forecast for weeks 2 to 13: incorporate any changes to expected payment dates, new invoices raised, updated payroll figures, and any material new information. This is not a full rebuild; it is an incremental update to the existing forward estimates.
- Identify the cash trough: what is the lowest closing balance across the 13-week period? Which week does it fall in? Is it above the minimum liquidity threshold?
- Produce the board report: a single page showing the closing cash balance by week as a chart, the cash trough date and amount, headroom above the minimum threshold, and any material items flagged for board attention.
How to Present the Model to the Board
The board does not need to see the full model. They need three things: the current cash position, the projected cash trough for the coming 13 weeks, and the headroom above the minimum liquidity threshold at that trough point.
The minimum liquidity threshold — the level below which the projected trough should not fall before triggering a board escalation — is typically set at three months of operating expenses. For a company burning £300,000 per month, the threshold is £900,000. If the 13-week forecast shows the trough will be below £900,000, management must present a plan to address the gap before the next board meeting.
The Early Warning System
The 13-week model is most valuable as an early warning system, not as a reporting tool. The difference is in how it is used: a reporting tool tells you what happened; an early warning system tells you what will happen with sufficient lead time to change it.
For the early warning function to work, the model must be maintained weekly without fail, the escalation trigger must be defined and agreed by the board in advance, and the CFO must have the authority and willingness to escalate without waiting for a board meeting. A cash crisis that appears in the forecast in week 1 of the 13-week window gives management 13 weeks to respond. The same crisis that appears in week 9 of the window gives them four weeks. The cost of remediation increases exponentially as the lead time decreases.
Key Takeaways
- The 13-week forecast is the right horizon: long enough to see structural problems, short enough to keep outer-week estimates grounded in reality.
- Use the receipts and payments method: forecast actual cash in and out of the bank account, week by week. Do not use the indirect method for short-term operational forecasting.
- The most predictable lines (payroll, loan repayments, VAT, corporation tax) should be entered with exact dates and amounts. The less predictable lines (customer collections) require DSO and DPO-based estimation.
- Update the model weekly: roll forward one week, populate actuals, explain material variances, re-forecast weeks 2 to 13.
- The board needs one number above all others: the cash headroom above the minimum liquidity threshold at the projected trough. If that number goes below zero, the company needs to act immediately.
- The minimum liquidity threshold (the escalation trigger) should be set at three months of operating expenses and agreed by the board in the treasury policy.
