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13-Week Cash Flow Forecast for a Small Company: How It Works, With an Example

By Di Ma, Project Manager 22 OCT 2026 · 6 MIN READ

A profit and loss statement tells you whether the business made money last month. It does not tell you whether you can pay salaries on the last Friday of next month. That is a different question, about timing, and the standard tool for it is the 13-week cash flow forecast: the next quarter, one column per week, showing the cash you expect to come in, the cash you expect to go out, and the balance at the end of each week.

Banks and lenders ask for one when a company is under pressure. But it is just as useful when nothing is wrong — it is how you see a tight week coming a month before it arrives, while there is still time to chase a customer or move a payment.

Why 13 weeks, and why weekly?

Thirteen weeks is one quarter. It is long enough to see beyond the next payroll and the next rent cheque, and short enough that the numbers are mostly known: invoices already sent, bills already received, salaries already agreed. Beyond about three months a forecast turns into a budget — useful, but built on assumptions rather than on documents.

Weekly columns matter because cash problems hide inside months. A month can end with a healthy balance and still dip below zero in its second week, because the big customer pays on the 25th and the supplier is due on the 10th. A monthly forecast averages that away; a weekly one shows it.

What goes into each week

A 13-week forecast uses the direct method: actual expected receipts and payments, not profit adjusted for accruals. For a typical small company the lines are:

• Opening cash — the bank balances today, across every account, converted into one currency.
• Money in from customers — open sales invoices, placed in the week each one is expected to be paid.
• Other money in — loan drawdowns, tax refunds, a shareholder top-up, if they are certain.
• Supplier bills — open purchase invoices, in the week they fall due.
• Payroll — net salaries on pay day, and the mandatory contributions on their own date (in Hong Kong, MPF contributions are due by the 10th of the following month).
• Regular payments — rent, subscriptions, insurance, utilities: anything that recurs on a predictable day.
• Loan instalments and one-off items — repayments, a tax payment, an equipment purchase you have committed to.
• Closing cash — opening cash plus money in minus money out, which becomes next week's opening cash.

The single most important number on the page is the lowest closing balance across the 13 weeks, and which week it falls in.

A worked example

Take a trading company with HK$420,000 in the bank on Monday of week 1. It has two open customer invoices: HK$180,000 due in week 3 and HK$95,000 due in week 6. It owes a supplier HK$260,000, due in week 2. Salaries of HK$70,000 go out at the end of every month (weeks 4, 8 and 13 here), rent of HK$28,000 on the 1st (weeks 1, 5 and 9).

Week 1 closes at HK$392,000 after rent. Week 2 the supplier is paid: HK$132,000. If the HK$180,000 invoice arrives on time in week 3, the balance recovers to HK$312,000 — fine. But the forecast should ask a sharper question: does this customer pay on the due date? If their last four invoices were all paid about three weeks late, that money really arrives in week 6, and weeks 3 to 5 run on HK$132,000 while salaries and rent leave in weeks 4 and 5. The lowest point drops to about HK$34,000 in week 5. Same invoices, same bills — a very different quarter.

The mistakes that make a forecast wrong

• Using due dates for customers who pay late. Due dates are what the invoice says; payment history is what happens. Place each receipt using that customer's own track record.
• Forgetting the non-monthly items. Annual insurance, the business registration fee, the profits tax payment, a bonus month. They are the ones that create the surprise.
• Counting your own transfers as income. Moving money from the USD account to the HKD account is not cash in. Forecast the group of accounts together, net of transfers between them.
• Mixing currencies. A USD balance and an HKD balance cannot simply be added. Convert at a stated rate, and remember the rate will move.
• Building it once. A forecast is only useful if it is rolled forward every week: last week's actuals replace the estimate, and a new week 13 is added.
• Starting from stale books. If the ledger is two months behind, the "open invoices" list is wrong before you begin. The forecast can only be as current as the bookkeeping under it.

Building one in a spreadsheet

1. Thirteen columns, each headed with the Monday that starts the week.
2. Opening cash in week 1: today's balance of every bank and payment account, in one currency.
3. List open sales invoices with their expected payment week (due date adjusted for how late that customer usually pays).
4. List open supplier bills with their due week.
5. Add payroll, statutory contributions, rent and every recurring payment on its usual day.
6. Add loans, tax and committed one-offs.
7. Closing cash = opening + in − out; carry it forward. Highlight the lowest week.
8. Every Monday: replace last week with actuals, add a new week 13, and look at the lowest point again.

Steps 3 to 6 are where the time goes, because they mean pulling invoices, bills and bank data out of the books by hand. That is the part worth automating.

How Certanta builds it from your books

Certanta clients have a 13-week forecast in their client portal, rebuilt from the ledger every time the books are updated — no spreadsheet to maintain:

• Cash today comes from every bank and payment account in the books, translated into the company's currency — HKD and USD accounts side by side, in the example below.
• Customer invoices sit on their due date. If one is already overdue, it is placed at that customer's usual delay — worked out from how late they paid their past invoices, or 30 days if there is no history yet — and flagged when even that date has passed.
• Bills sit on their due date; overdue ones land in week 1 so they are not forgotten.
• Payroll appears on the last working day of each month, with MPF on the 10th of the next.
• Regular payments — rent, subscriptions and the like — are spotted from the bank history, and only go into the forecast once your bookkeeper has confirmed them.
• Every line keeps its source, so you can open a week and see exactly which invoice or bill makes it up — and the page shows the lowest balance and the week it falls in.

The forecast is only as good as the books under it, which is why the rest of the service matters: bank lines arrive daily from connected accounts (or monthly from uploaded statements for traditional banks), every entry is checked by a bookkeeper, and transfers between your own accounts are matched so they never show up as income.

Frequently asked questions

This article is general information about cash flow forecasting, not financial advice for your company.