BAS calculator. What you owe the ATO this quarter, in the ATO's own labels. GST collected, GST paid, the difference, plus the PAYG on top.
Australian figures, checked 19 September 2026. Nothing you type leaves your browser.
The quarter, in four lines.
A business activity statement is mostly one subtraction. The GST you collected on sales, minus the GST you paid on purchases, is what you owe. If you paid more than you collected, which happens in a quarter you bought a ute, it's a refund. PAYG withholding is the tax you kept back from staff wages, and it's added on top. A PAYG instalment, if the ATO has set one, is a prepayment of your own income tax.
The calculator uses the one-eleventh rule the ATO uses: GST-inclusive amount divided by 11. Round to whole dollars on the form. And keep the receipts; the numbers are easy, the evidence is what an audit asks for.
The labels.
G1
Total sales for the quarter, GST included. GST-free sales go in here too, then come out of the GST calculation.
1A
GST on sales. One eleventh of taxable sales. What you collected on the ATO's behalf.
1B
GST on purchases. One eleventh of purchases that had GST in them. Claimed back.
W1 and W2
Total wages, and the tax withheld from them. Only if you employ people.
5A
PAYG income tax instalment, if the ATO has printed one on your statement.
Due dates
28 October, 28 February, 28 April, 28 July. About four weeks longer through an agent.
Or never do a BAS at ten at night.
The BAS is only hard when the quarter's invoices and receipts aren't in the system yet. When invoices write themselves and supplier bills get read and coded as they arrive, the BAS is a button in Xero on the 28th. That's the boring, valuable bit of what we build for small businesses. The GST calculator is here for the single-invoice version of the sum.
Questions people ask.
How is the GST on a BAS calculated?
GST on sales (label 1A) is one eleventh of your GST-inclusive taxable sales. GST on purchases (1B) is one eleventh of your GST-inclusive business purchases that had GST on them. 1A minus 1B is the net GST: pay it if it's positive, it's refunded if it's negative.
What do the BAS labels mean?
G1 is total sales. 1A is GST on those sales. G11 is non-capital purchases and G10 capital purchases, both feeding 1B, GST on purchases. W1 is total wages, W2 the tax withheld from them. 5A is your PAYG income tax instalment. On Simpler BAS, which most small businesses are on, you only report G1, 1A and 1B for GST.
When is my BAS due?
Quarterly BAS is due on the 28th of the month after the quarter ends: 28 October, 28 February, 28 April and 28 July. If you lodge through a registered agent you usually get about four extra weeks. Monthly lodgers are due on the 21st of the following month.
What purchases can't I claim GST on?
Anything without GST in it: wages, superannuation, bank fees, interest, most insurance stamp duty, council rates, water, and purchases from suppliers who aren't registered for GST. Also anything private, and only the business share of anything mixed, like the car.
Cash or accruals?
If you're on a cash basis, count sales when you were paid and purchases when you paid. On accruals, count them when invoiced. Businesses under $10 million turnover can choose; most small businesses use cash because it matches the bank account.
Is this calculator enough to lodge my BAS?
It gives you the GST figures and the total for a straightforward quarter. Imports, adjustments for bad debts, fuel tax credits, luxury car tax and wine equalisation tax aren't here. Your accounting software will have the same numbers with the receipts behind them, and a BAS agent can lodge for you.
Photo to come: Cal.
Calum Buchanan.
Melbourne. Ran the systems behind a services business, the phones, the invoices, the marketing, the reporting, and spent the last few years handing them to AI that works. This is that, for yours.