How to Calculate GST in Australia

The formulas for adding and removing 10% GST, why the GST in a total is one eleventh, how rounding works, and the mistakes that cause wrong totals.

The GST formulas

Goods and services tax (GST) in Australia is 10% on most goods, services and other items sold or consumed in Australia. All the arithmetic comes down to four short calculations.

“Excluding GST” is also written ex GST or GST-exclusive, and “including GST” is inc GST or GST-inclusive. When a quote says “plus GST”, the price is the ex-GST amount.

Worked examples

Adding GST

A job is quoted at $80.00 plus GST. The GST is $80.00 × 0.1 = $8.00 and the total is $80.00 + $8.00 = $88.00. Multiplying by 1.1 gives the same total in one step.

Taking GST out

A receipt shows a total of $88.00 including GST. The GST is $88.00 ÷ 11 = $8.00, so the price before GST was $88.00 − $8.00 = $80.00.

Rounding

A $250.00 total including GST contains $250.00 ÷ 11 = $22.7272…, which rounds to $22.73. The price before GST is $250.00 − $22.73 = $227.27. Taking the pre-GST price from the total, rather than dividing by 1.1 and rounding again, keeps the two parts adding up to exactly $250.00.

You can try any of these in the GST calculator.

Why the GST in a total is one eleventh

Call the price before GST P. The GST is 10% of P, which is 0.1 × P, so the price including GST is 1.1 × P. The share of the total that is GST is 0.1 × P ÷ (1.1 × P) = 0.1 ÷ 1.1 = 1/11. The P cancels, so the GST is one eleventh of any price that includes 10% GST, whatever the amount.

The same reasoning gives 3/23 for New Zealand’s 15% GST: 0.15 ÷ 1.15 = 15/115 = 3/23. See the NZ GST calculator.

Common mistakes

• Taking 10% off a GST-inclusive price. 10% of $110.00 is $11.00, which would leave $99.00. The GST is $10.00 and the price before GST is $100.00.
• Dividing by 10 instead of 11. The same error in another form. Divide a GST-inclusive price by 11 to find the GST.
• Adding GST twice. Multiplying a price that already includes GST by 1.1 gives $121.00 from $110.00. Check whether the price says “inc GST”, “incl. GST” or “plus GST”.
• Charging GST on GST-free items. Most basic food, for example, has no GST in the price. Work out the GST only for the taxable items. The invoice GST calculator keeps them separate.
• Rounding at each step. Round the GST once, at the end of the calculation, and take the other figure from the total.
• Rounding BAS amounts the wrong way. A BAS takes whole dollars, rounded down, and has its own rules for which sales and purchases to include. See the BAS calculator.

GST formulas for Excel and Google Sheets

With the price in cell A2, these formulas work in both programs. ROUND(…, 2) rounds to cents, so the stored value is the rounded one and not just a formatted display.

Use the first two formulas when the price in A2 excludes GST and the last two when it includes GST. For a long list of invoice lines, round the GST per line or on the total, in line with the ATO’s two rounding rules, and use the same rule consistently.

Which sales carry GST

The 10% applies to taxable sales. The ATO groups everything else into two kinds of sale where there is no GST in the price. “Credits claimable” means whether the seller can claim GST credits for the GST in the price of what it bought to make those sales.

The ATO publishes a GST food list and further pages on GST-free sales and input-taxed sales. If you are not sure how an item is treated, check there or ask a registered tax or BAS agent before you charge or claim.

Who has to register for GST

Only some businesses need to be registered. According to the ATO you must register for GST when:

• your business or enterprise has a GST turnover of $75,000 or more, or is expected to reach that (the ATO looks at the current month and the previous 11, and at the current month and the next 11);
• your non-profit organisation has a GST turnover of $150,000 a year or more;
• you provide taxi or limousine travel, including ride-sourcing, whatever your turnover is.

GST turnover is your total business income (not profit), less certain amounts such as GST included in sales to customers and input-taxed sales. You have 21 days to register once you are required to, and you need an Australian business number (ABN) first. Otherwise registering is optional, but if you register voluntarily you generally have to stay registered for at least 12 months. Registered businesses include GST in the price of taxable sales, claim credits for the GST in what they buy for the business, and report both on their activity statement.

Tax invoices

If a customer asks for a tax invoice, you must provide one within 28 days, unless the sale is $82.50 (including GST) or less. The invoice has to show the GST payable, which can be shown as an amount or, when the GST is exactly 1/11 of the total price, as the words “Total price includes GST”. The invoice GST calculator page lists the seven required details.

General information only, not tax advice. Check your situation with the ATO or a registered tax or BAS agent.

Last updated: 8 October 2026