The 13-week cash flow forecast: a simple tool that prevents nasty surprises

Most cash flow crises are visible weeks in advance to anyone who looks. The big BAS bill, the quiet month after a busy one, the large customer who always pays late, the annual insurance renewal: none of these are surprises in hindsight. A 13-week cash flow forecast is simply the habit of looking. It takes an hour to build and fifteen minutes a week to maintain, and it turns panicked phone calls into planned decisions.

What a 13-week forecast is

It is a table with thirteen columns, one for each of the next thirteen weeks, and rows for every type of cash coming in and going out. It starts with today’s bank balance, adds expected receipts, subtracts expected payments and carries the closing balance forward to the next week. By the end you can see, week by week, where the bank account is heading.

It is different from a profit and loss forecast. Profit forecasts record sales when they are made and costs when they are incurred. A cash flow forecast records money when it actually moves. That difference is the whole point: a profitable business can still run out of cash if the timing is wrong. Our guide to profit versus cash flow explains why.

Why thirteen weeks

Thirteen weeks is one quarter. It captures a full BAS cycle, most customer payment cycles and the timing of quarterly costs. It is also short enough that your estimates are grounded in known work, booked orders and real invoices rather than guesses about the distant future. Longer forecasts are useful for planning, but the 13-week view is the one that prevents crises.

Step one: start with today’s balance

Use your actual bank balance today, across all business accounts you use for operations. If you hold GST and PAYG withholding in a separate tax account, keep it separate in the forecast too, so you can see whether it is on track to cover the next BAS.

Step two: list cash coming in

Include every source of cash you expect, in the week you expect it to land, not the week it is due:

• Customer receipts. Start with your aged receivables: who owes you, how much and when they usually pay. If a customer always pays at 45 days on 30-day terms, use 45.
• New sales. For work you expect to complete and invoice, estimate when payment will actually arrive.
• Card settlements for retail and hospitality, based on typical weekly takings.
• Other receipts, such as tax refunds, asset sales or grant payments, only if confirmed.

Be realistic. Optimistic receipts are the most common forecasting error.

Step three: list cash going out

Include everything, in the week it will leave the account:

• Wages, PAYG withholding and, from 1 July 2026, super with every pay run.
• Supplier payments, based on your aged payables and usual terms.
• Rent, utilities, insurance, subscriptions and software.
• Loan, lease and finance repayments.
• BAS and any ATO payment plan instalments.
• Known one-offs: renewals, repairs, equipment, annual fees.

Your bank statements for the past three months are the best source for regular outgoings you might otherwise forget.

Seeing a gap in your forecast? Tell us about it in a 60-second enquiry. No credit check to enquire.

Step four: calculate the running balance

For each week, closing balance equals opening balance plus receipts minus payments. Carry the closing balance forward as the next week’s opening balance. Highlight any week where the balance falls below zero, or below the minimum buffer you want to keep.

Step five: read what it tells you

Three numbers matter most:

1. The lowest balance and the week it occurs.
2. The first week below zero, if any. This is your deadline.
3. The size of the gap at its deepest point.

If the lowest point is comfortably positive, you are in good shape. If it dips below zero, you know how much and when, and you have time to act. The cash flow gap calculator gives a quick version of this if you want a starting point.

Step six: act on it

With a gap in view, you can choose the cheapest fix:

• Chase customers who are due to pay before the low point.
• Bring forward invoicing on completed work.
• Ask a supplier to split or delay a large payment.
• Move discretionary spending to after the low point.
• Arrange a line of credit or short-term funding sized to the gap.

Our guide on fixing cash flow without borrowing lists more options. If funding is needed, see cash flow gap funding.

Step seven: update weekly

Each week, replace the oldest column with actual figures, add a new week thirteen and revise estimates based on what you now know. Compare forecast with actual: if receipts are consistently later than you predicted, adjust your assumptions. Within a month or two, the forecast becomes surprisingly accurate.

Common mistakes

• Using invoice terms instead of real payment behaviour.
• Forgetting irregular costs such as insurance, rego and annual subscriptions.
• Ignoring GST and PAYG withholding until the BAS is due.
• Counting unconfirmed income, such as a hoped-for contract.
• Building it once and never updating it.

Scenarios: what if

Once the forecast is built, test a few scenarios. What if your largest customer pays two weeks late? What if sales drop by a fifth for a month? What if a key machine breaks down? Running these takes minutes and shows how much buffer the business needs. Many owners use the worst reasonable scenario to size a line of credit, so it is there when needed and costs little when it is not.

Using your forecast with a lender

A simple forecast makes a funding request stronger. It shows the lender the size and timing of the gap, the cause and how it will be repaid. It also shows that the business is well managed. You do not need a polished document; a clear spreadsheet and a short explanation are enough.

Who should own the forecast

In a small business, the owner usually owns the forecast, even if a bookkeeper helps maintain it. The person who knows which customers are slow, which jobs are about to finish and which bills are negotiable is best placed to keep it realistic. If a bookkeeper or accountant updates the numbers, set a short weekly review together so the forecast reflects what is really happening in the business rather than just what is in the accounting system.

An illustrative example

An illustrative landscaping business builds its first 13-week forecast in August. It shows the account dipping to around minus $22k in week seven, when a large council invoice is due to be paid but the BAS and a machinery repayment fall in the same week. The owner calls the council’s accounts team and confirms the payment date, asks the equipment financier to move one repayment by a fortnight, and arranges a modest line of credit as a buffer. Week seven passes without a dishonour.

Plan the next quarter with confidence

If your forecast shows a gap you cannot close through timing alone, tell us its size and cause. There is no credit check to enquire, your details are not passed to a queue of lenders, and a real person reads your enquiry. Accurate figures from your forecast help us size the funding precisely. Apply now.