Add GST to a GST-exclusive amount
For a taxable supply at the standard Australian rate, GST is 10% of the GST-exclusive value. A A$100 net price produces A$10 GST and A$110 gross. The calculation is gross = net × 1.10.
Extract GST from a GST-inclusive amount
When a price already includes GST, the GST is one-eleventh of the total. A A$110 gross price contains A$10 GST, not A$11. The net amount is gross ÷ 1.10 and the GST component is gross ÷ 11.
This difference matters because the GST-exclusive amount is the smaller base on which 10% was originally added. Subtracting 10% from the larger gross number does not undo the addition.
Worked examples
| Starting point | Net | GST | Gross |
|---|---|---|---|
| A$100 GST exclusive | A$100.00 | A$10.00 | A$110.00 |
| A$550 GST inclusive | A$500.00 | A$50.00 | A$550.00 |
Arithmetic is not GST classification
Australia has taxable, GST-free and input-taxed sales. A supply can also contain components with different treatment. The calculator does not know what was sold, whether the seller is registered, whether an input tax credit is available or how a mixed supply should be apportioned.
Use the result only after establishing that the 10% taxable-supply treatment applies. Invoices and accounting systems can also have rounding conventions that produce small differences across multiple line items.
Official source
The ATO describes GST as a broad-based tax of 10% on most goods, services and other items sold or consumed in Australia. The legally binding GST ruling explains that GST on a taxable supply is 10% of its value and equivalent to 1/11 of its GST-inclusive price. See ATO: How GST works.