How to build a 13-week cash flow forecast for your business

Monthly forecasts tell you whether the quarter adds up. They don’t tell you that on the second Wednesday of November your BAS, your rent and a fortnightly payroll all leave the account within 48 hours, while your biggest customer pays on the last Friday of the month. A 13-week forecast does. It’s the tool turnaround accountants reach for first, and there’s no reason a small business owner can’t build one in an afternoon.

This guide walks through the layout, line by line, with a full worked example at the end. If you need a 12-month version for a loan application, our guide to a cash flow forecast for a business loan covers that format.

What makes a 13-week forecast different?

Three things set it apart from a budget or an annual forecast:

• It’s weekly. Each column is one week, usually ending on a Friday or Sunday. That’s fine enough to show clashes between big payments.
• It’s pure cash. Nothing goes in until it moves through the bank. A sale invoiced today but paid in five weeks appears in week five.
• It rolls. Every week you drop the week just gone and add a new week thirteen. The forecast never runs out, and it gets more accurate the longer you keep it.

Thirteen weeks is a full quarter, long enough to see a gap coming with time to act, and short enough that most of the numbers come from invoices, bills and rosters you already have, not guesses.

What do you need before you start?

Gather four things. It takes less time than it sounds.

1. Today’s cleared bank balance, across every business account you use for operating costs. Leave out a dedicated tax account if you keep one, or include it and show the transfers; just be consistent.
2. Your aged debtors report, listing every unpaid invoice and how overdue it is.
3. Your unpaid bills, plus a list of regular payments: wages and pay dates, rent, loan and lease repayments, insurance, subscriptions.
4. Your tax calendar: BAS due dates, PAYG instalments and super timing.

How do you lay out the sheet?

Put weeks across the top and line items down the side. A simple structure works best:

Row	What goes in it
Opening balance	Week 1: today’s bank balance. Later weeks: the previous week’s closing balance
Receipts	One row per major customer, a row for smaller customers combined, and a row for other income
Payments	Wages (net), PAYG withheld (if paid separately), super, rent, suppliers, loan and lease repayments, BAS/GST, other
Net cash flow	Total receipts minus total payments
Closing balance	Opening balance plus net cash flow
Minimum comfortable balance	The level below which you’d start to worry, as a fixed line

That final row is the one most people leave out, and it’s the most useful. If your closing balance dips below it, the cell should turn red. Your eye goes straight to the problem week.

How do you forecast receipts realistically?

Here’s the usual weak spot. Owners enter invoices in the week they fall due. Customers pay when they pay.

Go through the debtors report customer by customer and ask: when does this one actually pay? If a builder on 30-day terms has paid on day 52 for the past six months, put their invoices in the week that matches day 52. If a customer pays on the last business day of the month regardless of terms, use that.

National data backs up this caution. Under the Payment Times Reporting Scheme, the measure of how long big companies take to settle 95% of small-supplier invoices stretched from 58 to 64 days for January to June 2025. Averages hide a long, slow tail.

For future sales not yet invoiced, use your history: what came in during the same weeks last year, adjusted for anything you know has changed. Be conservative. A forecast that’s pleasantly surprised is better than one that lets you down.

Which payments do people forget?

Regular costs are easy. The lumpy ones cause the trouble. Check every one of these is in the right week:

• Quarterly BAS. If you lodge quarterly, the September quarter falls due on 28 October, January–March is due 28 April and April–June is due 28 July. The October–December quarter gets extra time, to 28 February. Tax agents can sometimes lodge later under their program, so confirm your actual dates.
• Super, every pay cycle. Under Payday Super (from 1 July 2026), each pay run’s contributions generally have to land in staff funds inside seven business days. If you pay weekly, that’s a super line every week, not a lump each quarter.
• PAYG instalments on your income tax, if you’re in the system.
• Annual and quarterly bills: insurance renewals, vehicle registrations, workers’ compensation premiums, accounting fees.
• Leave loading and Christmas pay, when staff take leave and pay runs shift.
• Loan repayments on their actual debit dates, not their monthly totals.
• Your own drawings. If you take money out, it’s a payment.

Here’s a trap worth knowing about. Because the December quarter BAS isn’t due until late February, a forecast built in early November covers weeks with no BAS at all. January looks clean. Then, in the weeks immediately after the window, February brings that BAS on top of a slow post-holiday month. Rolling weekly solves it, because February moves into view in late November. But if you only look once, you’ll miss it.

Seeing a gap you can’t close by moving dates around? Tell us roughly what you need and when; someone will call to talk it through, and enquiring doesn’t touch your credit file.

A worked example: a joinery business heading into Christmas

This is an illustration with made-up figures.

A Geelong joinery business with twelve staff builds its forecast on Monday 26 October 2026. Opening balance is $18,000. The quarterly BAS of $24,000 is due on Wednesday 28 October. The owners have also committed to a $15,000 deposit on a new edge bander in the week of 30 November. The workshop closes for two weeks over Christmas, with leave paid in advance in the week of 14 December.

Week starting	Receipts	Wages + super	Other payments	Net	Closing
26 Oct	$38,000	$20,700	$33,000	−$15,700	$2,300
2 Nov	$31,000	$20,700	$14,300	−$4,000	−$1,700
9 Nov	$46,000	$20,700	$11,900	$13,400	$11,700
16 Nov	$29,000	$20,700	$7,000	$1,300	$13,000
23 Nov	$41,000	$20,700	$9,500	$10,800	$23,800
30 Nov	$52,000	$20,700	$32,000	−$700	$23,100
7 Dec	$35,000	$20,700	$11,100	$3,200	$26,300
14 Dec	$30,000	$30,200	$6,000	−$6,200	$20,100
21 Dec	$6,000	$10,100	$1,500	−$5,600	$14,500
28 Dec	$3,000	$10,100	$500	−$7,600	$6,900
4 Jan	$24,000	$20,700	$13,300	−$10,000	−$3,100
11 Jan	$41,000	$20,700	$11,100	$9,200	$6,100
18 Jan	$39,000	$20,700	$9,000	$9,300	$15,400

“Other payments” covers rent, timber and hardware suppliers, a monthly equipment loan repayment, the BAS in week one and the machine deposit in the week of 30 November.

The forecast reveals two problems, both small and both fixable. Week two goes $1,700 into the red because the BAS lands before a large cabinet installation is paid. Week eleven goes $3,100 under because January rent and supplier bills arrive while customers are still returning from holidays.

The owners act on both in October. They call the cabinet customer and agree a progress invoice, paid within the week. They move the edge bander deposit to mid-January, after the first big receipts. And because the February BAS will follow straight after week thirteen, they arrange a small overdraft limit with their bank before Christmas, while the November figures look strong. None of it would have been visible on a monthly forecast, which showed a comfortable positive balance in every month.

How do you keep it accurate week to week?

The forecast becomes valuable only through the weekly update. Keep it to a fixed time and a fixed routine:

1. Replace week one’s forecast figures with actuals from the bank statement.
2. Note the big differences in a comments column. A customer paid late, a bill came early, sales beat the estimate.
3. Delete the completed week and add a new week thirteen.
4. Adjust the assumptions the differences point to. If a customer keeps paying on day 60, move all their invoices.
5. Check the lowest closing balance and decide what, if anything, to do about it.

After six to eight weeks of this, most owners find the first four weeks of the forecast are accurate to within a few thousand dollars. That’s when it becomes a genuine decision tool, not a spreadsheet you fill in out of duty. For the broader weekly routine around it, see how to manage business cash flow.

What should you do when the forecast shows a gap?

Work through the free options first, roughly in this order:

• Bring receipts forward: invoice sooner, ask for progress payments, follow up overdue accounts by phone.
• Push discretionary payments back: equipment, stock top-ups, non-urgent purchases.
• Talk to suppliers about timing before a bill is overdue, not after.
• If the BAS can’t be covered in full, ring the ATO before the due date, and lodge on time either way.

If a gap remains, size it precisely. The forecast tells you the amount, the week it starts and the week the cash comes back, the three answers any lender will ask for. The cash flow gap calculator helps turn the shortfall into a sensible facility size. Banks are the natural first call for an overdraft or line of credit if you meet their criteria. When a bank can’t help, or can’t help in time, options for covering a cash flow gap include non-bank lines of credit, short-term working capital loans and invoice finance. If the gap is already here rather than weeks away, start with how to fix a cash flow problem fast.

Your forecast found a gap. Let’s look at closing it

A 13-week forecast that shows a dip is a forecast doing its job. You now know how much, when and for how long, which puts you in a much stronger position than most owners who come looking for finance.

Bring those numbers to us. Enquiring takes about a minute, and your credit score stays exactly where it is. We don’t forward your enquiry to a whole crowd of lenders; it’s read by one person on our team, who then calls you to discuss what fits. Fill the form in with the real figures from your forecast, because accurate numbers mean we can suggest the right option without any back and forth.

Show us your gap and see what fits →