Profit tells you whether your business works. Cash tells you whether it survives the next quarter. Plenty of profitable companies have run into trouble because a large customer paid late in the same month that payroll, rent and an annual software renewal all landed. A 13-week cash flow forecast is the tool finance teams use to see that kind of month before it arrives.

Thirteen weeks is one quarter. It is long enough to show the effect of a slow-paying customer or a new hire, and short enough that you can make each week reasonably accurate. Here is how to build one, even if you have never forecast before.

Start with the cash you have today

Your forecast opens with a single number: the total cash you can actually spend this week. Add up the balances of every operating account, but leave out money that is already committed, such as sales tax you have collected and owe to the state, or a deposit you are holding for a customer. If you hold cash in several banks, list each balance on its own line so you can check them against statements later.

List the cash coming in each week

Next, estimate what will arrive in each of the thirteen weeks. The most reliable source is your list of open invoices. For each one, do not assume the customer pays on the due date. Look at how that customer has paid in the past and use their real average. If a client usually pays fifteen days late, move their invoice fifteen days to the right.

Then add income that is not invoiced yet but is very likely, such as monthly retainers, subscription renewals or card sales from your online store. Be conservative. A forecast that assumes every deal closes on time is worse than no forecast at all, because it gives you confidence you have not earned.

• Open invoices, shifted by each customer’s real payment habits

• Recurring revenue, such as retainers and software subscriptions

• Expected card or online sales, based on the same weeks last year

• One-off items such as a tax refund or a loan drawdown

List the cash going out

Outgoing cash is usually easier to predict than incoming cash, because you control most of it. Start with the fixed items: payroll dates, rent, loan repayments and insurance. Then add supplier bills you have already received, placed in the week you plan to pay them rather than the week they are due. Finally, add variable spend such as card purchases, using the average of the last three months.

Do not forget the irregular payments that catch teams out. Quarterly estimated taxes, annual software renewals, insurance premiums and bonuses all belong in the forecast, in the exact week they will leave your account.

Calculate the closing balance for each week

For each week, take the opening balance, add the cash coming in and subtract the cash going out. The result is that week’s closing balance, which becomes the opening balance for the following week. Thirteen rows later, the whole quarter is in view.

Now set a floor. This is the lowest balance you are comfortable holding, often one month of fixed costs. Any week that dips below the floor is a week that needs a decision today, while you still have options.

Act on what the forecast shows

A forecast is only useful if it changes what you do. When a week falls below your floor, you have a handful of levers, and the earlier you pull them the cheaper they are.

• Ask a large customer to pay a deposit or move to shorter payment terms

• Schedule a supplier bill for its due date instead of paying it early

• Delay a planned purchase or a new hire by a few weeks

• Arrange a line of credit while your numbers still look healthy

Keep it current every week

The forecast is a living document. Set a fixed time each week, such as Monday morning, to replace last week’s estimates with real numbers and roll the forecast forward by one week. Compare what you predicted with what happened. If you are consistently too optimistic about one customer or one cost, adjust your assumptions.

After a month or two, the weekly update takes fifteen minutes and becomes one of the most valuable habits in your business. In Ledgerly, the forecast builds itself from your bank balances, open invoices, scheduled bills and payroll, so the Monday update becomes a review of the numbers rather than a data-entry session. Whichever tool you use, the habit is what protects your runway.