13-Week Cash Flow Forecast: Build One for a Small Business

A 13-week cash flow forecast is a week-by-week list of the money you expect to come in and go out over the next quarter, with a running bank balance. It's short enough to be accurate and long enough to warn you about a tight week while you can still do something about it. Below is a worked example you can copy, plus the habits that keep it honest.

Why 13 weeks and not 12 months?

A 12-month forecast is useful for planning a loan or a hire. But beyond about three months, most small business numbers are educated guesses. You don't know which jobs you'll win in Mar 2027.

Thirteen weeks is one quarter. Most of what lands in that window is already visible: invoices you've sent, bills you've received, wages, rent, loan repayments, the quarterly tax payment. That makes the forecast mostly fact with a little estimate, rather than the other way round.

Weekly buckets matter too. A monthly forecast can show a healthy month-end balance while hiding the Tuesday when wages and the supplier run both go out before the big customer pays.

What goes into it

You need three things per week: what you start with, what comes in, what goes out.

Cash in

• Customer invoices you've already sent, placed in the week you realistically expect payment (not the due date if that customer is always late)
• Expected new invoices for work booked in
• Card and EFTPOS settlements if you take payments over the counter
• Anything unusual: a refund, an asset sale, a loan drawdown

Cash out

• Wages and super (on your pay cycle)
• Rent, loan and lease repayments
• Supplier bills already received, by due date
• Regular costs: phone, software, insurance, fuel
• Quarterly or annual items: insurance premiums, registrations, the quarterly GST and tax instalment amounts your BAS agent or accountant gives you
• Owner drawings

Don't overthink categories. Five or six lines is plenty for most small businesses.

A worked example: an electrical contractor in Newcastle

Tom runs an electrical business with three employees. Wages go out fortnightly. He sat down on Tue 6 Oct 2026 with a reconciled bank balance of $18,500 and built this for the 13 weeks starting Mon 12 Oct 2026.

Check: $18,500 + $124,900 - $122,700 = $20,700. The closing balance in week 13 matches.

On its own, that looks comfortable. The lowest point is $17,100 in the week of 26 Oct 2026. Notice what happens over Christmas, though: receipts drop to $4,100 and then $1,800 while costs keep going. That's normal for a trade business, and a forecast makes it a known quantity instead of a nasty surprise in Jan 2027.

Now stress-test it

A forecast earns its keep when you ask "what if?". Tom's biggest customer, a property manager, owes $8,000 that he's pencilled in for week 2. They've paid three weeks late before.

So Tom moved that $8,000 from week 2 to week 5:

• Week 2 closes at $12,800 instead of $20,800
• Week 3 closes at $9,100 instead of $17,100
• Week 4 closes at $13,100 instead of $21,100
• Week 5 catches up and closes at $20,900, the same as before

A low point of $9,100 the week wages, the ute repayment and the quarterly GST all go out is a very different conversation. It's still positive, but there's not much room if anything else slips. Tom rang the property manager in week 1 rather than waiting, and booked a smaller supplier order for week 3.

That's the whole point. You can't change the past, but you can change week 3 when it's still two weeks away.

How to build yours in about an hour

1. Reconcile your bank account first. Start from a balance you trust. If your books are three weeks behind the bank, the forecast starts wrong.
2. Pull your aged receivables. Put each unpaid invoice in the week you actually expect the money. Be honest about slow payers.
3. Pull your aged payables. Put each bill in the week you plan to pay it.
4. Add the fixed stuff. Wages, super, rent, loans, subscriptions. These are the easiest lines to get right.
5. Add estimates for new work. Use what's booked, not what you hope for. If in doubt, leave it out.
6. Add the lumpy items. Insurance renewals, registrations, quarterly payments. These are the ones that ambush people.
7. Run the running balance and find the lowest week. That's your number to manage.

A spreadsheet is fine. You don't need special software for the forecast itself; you need accurate inputs.

Keeping it alive: the weekly 15 minutes

A forecast you built once and never touched is worse than none, because it gives you false comfort. Every Monday:

• [ ] Reconcile last week's bank transactions
• [ ] Replace last week's forecast with what actually happened
• [ ] Add a new week 13 on the end
• [ ] Move any invoice that's now overdue into a later week
• [ ] Check the lowest week and decide if you need to act

Over time you'll notice patterns. Some customers always pay on day 40. Your fuel estimate is always light. Fix the assumptions and the forecast gets sharper every quarter.

What to do when a week goes red

If a week drops below zero, or below the buffer you're comfortable with, you have options. Ranked roughly from easiest to hardest:

• Chase overdue invoices now, by phone, not just email
• Bring forward invoicing on finished jobs
• Push a non-urgent supplier payment by a week (ask them first; it keeps the relationship good)
• Ask for a deposit on the next large job
• Delay a planned equipment purchase
• Talk to your bank about an overdraft before you need it, not the week you need it

If the red weeks keep coming back quarter after quarter, that's a pricing or terms problem rather than a timing one, and it's worth sitting down with your accountant.

For more on why a profitable business can still hit a cash dip, see cash flow management software.

How HelloBooks helps

HelloBooks doesn't build the 13-week forecast for you, but it gives you the inputs. On the Free plan you get accounts receivable and payable ageing, a Cash Flow report, invoices and bills, and one live bank feed (or CSV statement import). The reconcile screen suggests a match for each statement line with a confidence score, so getting to a trusted starting balance each Monday is quicker.

On Pro (A$30/month) you also get recurring invoices, bills & approvals, AI Analysis on every report and Excel export, which makes it easy to drop your ageing reports straight into a forecast spreadsheet. Compare on the pricing page.

FAQs

What's the difference between a cash flow forecast and a budget?

A budget is a plan for profit: income and expenses for the year. A cash flow forecast tracks when money actually hits and leaves the bank. A budget can balance perfectly while the cash forecast shows a shortfall in a particular week.

Should I include GST in my forecast?

Yes. Your forecast is about real cash, so use GST-inclusive amounts for receipts and payments, and add the quarterly GST payment as its own line using the figure from your BAS agent or accountant.

How accurate does it need to be?

Accurate enough to spot the lowest week. Getting fixed costs right matters most. Receipts are always the uncertain part, so be conservative with them.

Is 13 weeks enough for a seasonal business?

It's enough for weekly management. Seasonal businesses usually keep a rougher 12-month view alongside it to plan for the quiet months, then use the 13-week forecast for the detail.

Do I need to reconcile before forecasting?

Ideally, yes. If your starting balance doesn't match the bank, every week after it is off by the same amount.

An hour to build it and fifteen minutes a week to keep it current. It's one of the best trades in small business admin.

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