How to build a 13-week cash flow forecast
A 13-week cash flow forecast lists expected receipts and payments week by week for one quarter, starting from today's bank balance. Build it from actual commitments rather than averages, roll it forward every week, and compare each week against what really happened.
The 13-week forecast is the standard instrument for managing cash when it is tight. Thirteen weeks is one quarter: far enough ahead to act on a problem, close enough that you can still name the individual payments.
Set it up#
Thirteen columns, one per week, starting with the current week. Rows for receipts, rows for payments, and three lines at the bottom: total in, total out, and closing balance. The opening balance of week one is the money in your accounts today.
Fill in receipts from real invoices#
Take your unpaid invoices and place each one in the week you genuinely expect it to be paid, not the week it is due. If a customer has taken sixty days on every invoice for two years, sixty days is your assumption regardless of your terms. Then add expected new sales in later weeks, on the same collection lag.
Fill in payments by category#
Payroll on its actual dates, with payroll taxes on theirs. Rent on the day it goes out. Supplier payments from your unpaid bills, in the week you intend to pay each one. VAT and any tax falling in the quarter. Loan repayments. Anything annual that happens to land in these thirteen weeks.
Read the bottom row first#
Find the lowest closing balance across the thirteen weeks and the week it falls in. That single number tells you whether you have a problem, how big it is, and how long you have to fix it. Everything else in the document is working.
Roll it forward every week#
Each week, drop the week that has passed, add a new week thirteen, and update the opening balance to what is actually in the bank. This is the habit that makes the forecast worth having, and it is the one people abandon first.
Track the variance, because that is the real output#
Before you roll forward, compare what you forecast for last week against what happened. Consistently over-forecasting receipts means your collection assumptions are too optimistic, and now you know by how much. After a month of this the forecast becomes noticeably more accurate, which no amount of care in the initial build will achieve.
What it is not for#
A 13-week forecast is built from commitments that already exist, so it cannot tell you anything about a decision you have not taken. Whether to hire, whether to raise, whether to open a second site: those need a longer model built from drivers rather than from your current invoice ledger. The two documents coexist and neither replaces the other.