How to build a 13-week cash flow forecast

If a growing business could have only one financial report, it should be this one. Not the P&L, not the balance sheet — a simple table showing how much cash you will have at the end of each of the next thirteen weeks.

Businesses rarely fail because they are unprofitable. They fail because they run out of cash while still profitable on paper. The 13-week forecast is the tool that sees that coming.

Why thirteen weeks

Thirteen weeks is one quarter, and it sits in a useful middle ground.

• Long enough to act. Spot a shortfall six weeks out and you can chase collections, delay a purchase, renegotiate terms or arrange a facility. Spot it two days out and you can only panic.
• Short enough to be accurate. You broadly know who owes you what over the next quarter. Beyond that, receipts become guesswork and weekly detail stops meaning anything.
• Weekly, not monthly. A month-end balance can look fine while hiding a week in the middle where payroll and GST fall together and the account goes negative.

The structure, line by line

Thirteen columns, one per week. The rows run like this:

Row	What goes in it
Opening cash	Last week’s closing balance, across all accounts
Customer receipts	Forecast by customer and expected date
Other receipts	Refunds, asset sales, interest, capital injections
Total receipts
Supplier payments	By supplier and due date, per your payment terms
Payroll	Salaries on the actual pay date
Rent and overheads	Fixed monthly payments on their real dates
GST and TDS	On the statutory due dates — see our due date calendar
Loan repayments	Principal and interest, on EMI dates
Capex	Committed capital spend
Total payments
Net cash flow	Receipts minus payments
Closing cash	Opening plus net flow
Facility headroom	Undrawn working capital limit
Cash floor	Your minimum acceptable balance

Every week where closing cash drops below the floor is highlighted. That highlight is the entire point of the exercise.

Building it from scratch

1. Start from actual cash, today

Reconciled bank balances across every account, as of today. Not the book balance — the bank balance. If your books and bank disagree, fix that first; a forecast built on an unreconciled number is wrong from row one.

2. Forecast receipts customer by customer

This is the part that decides whether the forecast is any good. Take your debtor ageing and, for each material customer, write down when you actually expect to be paid — based on how they have actually paid before, not on your invoice terms.

A customer on 30-day terms who consistently pays at 65 days goes in at 65 days. Forecasting receipts as a flat percentage of sales is the single most common reason these models fail.

3. Lay out payments on their real dates

Payroll on pay day. Rent on the 1st. GST and TDS on their due dates. Suppliers per the terms you actually honour. Loan EMIs on their dates. Precision here is easy, because you control most of it.

4. Set your cash floor

The minimum balance you are willing to hold — typically enough to cover a payroll cycle plus the next statutory payment. Anything below it is a problem to solve now.

5. Add the commitments nobody writes down

Annual insurance renewals, festival bonuses, advance tax instalments, a large purchase order already placed. These are what turn a comfortable forecast into a shortfall.

Six mistakes that make it useless

1. Forecasting from profit, not cash. Revenue is recognised when invoiced; cash arrives when paid. These can be months apart.
2. Optimistic collections. Using invoice terms instead of actual payment behaviour. Every forecast built this way is too rosy.
3. Monthly buckets. They hide the week in the middle where everything falls due together.
4. Building it once. A forecast not rolled weekly is stale within a fortnight.
5. No variance review. If you never compare forecast to actual, you never learn which assumptions are wrong.
6. Forgetting statutory payments. GST and TDS are large, fixed-date and non-negotiable. Leaving them out is how businesses get caught.

Running it every week

Twenty minutes every Monday, in this order:

1. Replace last week’s forecast with actuals.
2. Note the variance on receipts and payments — and why.
3. Roll forward: drop the oldest week, add week thirteen.
4. Update receipts for any customer whose behaviour changed.
5. Check every week against the floor.

After a few months the variance review becomes the most valuable part. You learn which customers are reliable, which suppliers flex, and how accurate your own instincts are.

What to do when a week goes red

A breach six weeks out is not a crisis. It is information, early enough to use. In roughly this order of speed and cost:

1. Accelerate collections — chase the specific overdue customers the forecast is relying on.
2. Re-sequence payments — move discretionary spend past the tight week.
3. Draw on the facility — if there is headroom, this is what it is for.
4. Negotiate terms — with a supplier, before you need to, not after.
5. Arrange funding — which takes weeks, which is why you need to see it coming.

Our cash runway calculator gives a quick top-line read, and the working capital calculator shows how many days of cash your operating cycle is absorbing — usually the biggest lever you have.

Building and running a 13-week forecast every week is a core part of our virtual CFO service. If you would rather see the tight week before it arrives without maintaining the spreadsheet yourself, talk to us.