GST calculator.

The two formulas

GST arithmetic at the 10% rate

You know	To find	Formula
Price excluding GST	GST amount	price × 0.1
Price excluding GST	Price including GST	price × 1.1
Price including GST	GST amount	total ÷ 11
Price including GST	Price excluding GST	total ÷ 1.1

The "divide by 11" rule trips people up. GST is 10% of the original price, and the total is 110% of it, so the GST is one-eleventh of the total — not one-tenth. On a $1,100 invoice the GST is $100, not $110.

Showing GST on a tax invoice

If you are registered for GST, a tax invoice for a sale over $82.50 (including GST) must show your identity and ABN, the date, a description of what was sold, the GST amount (or a statement that the total includes GST), and the extent to which each item is taxable. For sales of $1,000 or more it must also show the buyer's identity or ABN. Prices shown to consumers must be GST-inclusive under Australian Consumer Law.

Rounding

The ATO allows GST to be rounded to the nearest cent on each line or on the invoice total, as long as you are consistent. This calculator rounds the GST component to the nearest cent. When you multiply by a quantity, the GST is calculated on the line total so the invoice adds up.

Who needs to add GST?

Only GST-registered businesses charge GST. Registration is compulsory once your GST turnover reaches $75,000 in a 12-month period ($150,000 for not-for-profits), and from the first dollar if you drive a taxi, limousine or rideshare vehicle. Below the threshold registration is optional — it lets you claim GST credits on your costs, at the price of charging GST and lodging a BAS. Our GST registration guide covers the timing, and our BAS service handles the lodgments.

GST-free and input-taxed sales

Not everything attracts GST. Most basic food, many health and medical services, education, childcare and exports are GST-free: no GST is charged, but the seller can still claim credits on related costs. Residential rent, the sale of existing residential premises and most financial supplies are input-taxed: no GST is charged and no credits can be claimed. Getting the classification right matters more than the arithmetic — it is where most BAS errors start.

Sources: ATO — GST, Tax invoices, Registering for GST. General information only, not advice. SMT Accounting is not affiliated with the ATO.