BAS GST Calculator Australia

Every Australian small business registered for GST has the same quarterly chore: subtract one number from another and pay the difference to the ATO. The two numbers are 1A and 1B, the difference is the BAS GST line and the whole chore takes 30 minutes once you know where to find each figure in your books.

This post walks through the calculation, the two accounting methods that decide which figures end up in those cells, the simpler BAS rule that took the question count from seven to three, the lodgement deadlines that catch out new business owners and a fully worked quarterly example you can compare against your own.

What a BAS actually is

The Business Activity Statement is the form GST-registered businesses lodge to report and pay (or claim back) the GST that flowed through the business in a period. It also covers PAYG withholding for employees, PAYG instalments for the business owner’s income tax and a handful of smaller obligations like FBT instalments and fuel tax credits.

For the GST line, four cells matter:

• G1: total sales (GST-inclusive). Total revenue for the period, including the GST component on each invoice.
• 1A: GST on sales. GST you collected from customers. Usually 1A = G1 ÷ 11 if every sale was taxable.
• 1B: GST on purchases. GST you paid to suppliers and can claim back as a credit.
• Net BAS GST: 1A − 1B. The figure you remit or the ATO refunds for the period.

The core formula

Net BAS GST = 1A − 1B

If 1A is bigger than 1B, you remit the difference to the ATO. If 1B is bigger than 1A, the ATO refunds the difference.

A small business that took $44,000 in revenue (G1) and spent $11,000 on GST-inclusive expenses across the same quarter has:

• 1A = $44,000 ÷ 11 = $4,000
• 1B = $11,000 ÷ 11 = $1,000
• Net BAS GST = $4,000 − $1,000 = $3,000 owing to the ATO
• Cash to set aside each quarter on this run rate: roughly $3,000 plus a 10% buffer

The GST calculator on this site handles each of the divide-by-11 calculations in one tap. For repeat work across a whole quarter of receipts, the Reverse GST calculator is faster.

Cash vs accruals: which method applies

Two accounting methods decide which figures end up in 1A and 1B for the quarter. Choosing the wrong one is the most common BAS error in the first six months of a new business.

Cash accounting reports GST in the period the cash actually moved – when a customer paid you, or when you paid a supplier. Businesses with aggregated turnover under $10 million can elect cash basis under the ATO’s accounting method rules.

Non-cash (accruals) accounting reports GST in the period the invoice was issued, regardless of whether the money has changed hands. Businesses at or above $10 million turnover must use this method. A hybrid is possible for specific industries with the ATO’s permission, but it’s rare in practice.

The right answer for most small businesses is cash. It matches the way the bank account behaves, removes the timing mismatch between invoiced sales and paid sales and keeps the BAS figure inside the cash you actually have on hand.

The trade-off shows up on long-payment-cycle work. If you invoice in December and collect in February, an accruals-method business reports the GST in the December BAS; a cash-method business reports it in the March BAS. Cash is kinder to cashflow; accruals matches the books most accountants prefer to read.

Simpler BAS vs full reporting

Since 1 July 2017 the ATO has run a simplified version of the BAS for small businesses. The number of GST questions on the form dropped from seven to three. Most small businesses now lodge with three fields, not seven.

• Simpler BAS (default for businesses under $10 million GST turnover). Three fields: G1 (total sales), 1A (GST on sales), 1B (GST on purchases). Suppliers and software classify each transaction simply as ‘GST’ or ‘No GST’.
• Full reporting (for businesses at or above $10 million turnover). Seven fields including export sales (G2), other GST-free sales (G3), capital purchases (G10) and non-capital purchases (G11).

The ATO’s Simpler BAS GST bookkeeping guide is the official reference. It tells your accounting software which classifications to retire and which to keep – Xero, MYOB and QuickBooks all default to Simpler BAS for businesses under the threshold.

Lodgement frequency: quarterly, monthly or annual

The ATO assigns a lodgement frequency based on turnover and the business’s history. Quarterly is the default for most small businesses (GST turnover under $20 million). Monthly is compulsory for businesses with turnover at or above $20 million or for those the ATO believes pose a higher compliance risk, and is optional for any business that prefers a tighter feedback loop. Annual reporting is available to voluntarily registered businesses with GST turnover under $75,000 ($150,000 for non-profits) – the ones who registered without being required to – and only one BAS is due each year, on 31 October.

The full rules are on the ATO’s GST reporting methods page.

Quarterly lodgement deadlines

Quarterly BAS dates work the same way each year, with two deadlines depending on whether you self-lodge or use a registered tax or BAS agent.

• Q1 (July–September): 28 October if you self-lodge; up to 25 November with an agent.
• Q2 (October–December): 28 February (extended from the standard 28 days because of Christmas closures).
• Q3 (January–March): 28 April if you self-lodge; up to 26 May with an agent.
• Q4 (April–June): 28 July if you self-lodge; up to 25 August with an agent.

Lodging through a tax or BAS agent gets you roughly four extra weeks each quarter. For most small businesses the agent’s fee is recovered in the timing flexibility alone.

A worked quarterly example

A small Brisbane cafe with one full-time owner, two casual staff and turnover well under $10 million ran the following figures for a March quarter on cash-basis Simpler BAS:

• G1 (total GST-inclusive sales for the quarter): $176,000
• 1A: $176,000 ÷ 11 = $16,000
• GST-inclusive purchases (rent, supplier invoices, equipment): $66,000
• GST-free purchases (wages, super, basic food ingredients): $44,000 – no GST credit available
• 1B: $66,000 ÷ 11 = $6,000
• Net BAS GST: $16,000 − $6,000 = $10,000 owing

If the cafe lodges through a registered agent, the lodgement window closes 26 May and the $10,000 is debited shortly after. Self-lodging closes the window on 28 April, with the same payment timing.

The owner’s quarterly habit – setting aside one-eleventh of each daily takings figure into a separate ATO-savings account – is what keeps that $10,000 inside the business when the BAS lands. Without that habit the cafe would be drawing on operating cash to fund GST it has been holding for the ATO all quarter.

Common BAS GST mistakes

Five errors we see in the early quarters of a new business:

• Mixing methods between BAS lines. Recording sales on accruals and purchases on cash distorts both 1A and 1B. Pick one method and stick to it across both columns.
• Claiming GST credits on input-taxed purchases. Residential rent, financial-service fees and some bank charges are input-taxed. The supplier doesn’t charge GST and you can’t claim a credit. Software defaults sometimes get this wrong.
• Including GST-free sales in 1A. A bakery selling bread (GST-free) and pies (GST-inclusive) needs to split the takings before dividing by 11. Lumping the whole G1 figure into 1A over-states the GST owing and the ATO collects a refund-shaped follow-up call months later.
• Missing capital purchase allocations. Under full reporting, capital purchases sit in G10, not G11. Simpler BAS users don’t have to worry about this distinction, but businesses approaching the $10 million threshold do.
• Forgetting the lodgement window. Late BAS attracts a failure-to-lodge penalty (currently $330 per 28 days late, capped at five units), plus general interest charge on any GST owing. The penalty is a flat fee – it doesn’t scale with the size of the business.

One last thing

If you’re new to BAS, set up two habits before your first lodgement. First, run a clean trial balance from your accounting software at the end of every month – not just at the end of the quarter – so the BAS calculation is a check rather than a discovery. Second, drop one-eleventh of every customer payment into a separate ATO-savings sub-account as it lands. The ATO’s interactive GST calculation worksheet handles the actual maths once the figures are clean, and the Reverse GST calculator is the spot-check tool for individual receipts that look off against the system.

The calculators and guides on gstcalculator.net.au are for general information only and do not constitute tax, financial, or legal advice. Consult a registered tax agent for advice specific to your situation.