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To add GST, multiply the GST-exclusive price by 1 + the GST rate. To remove GST from a GST-inclusive price, divide the total by 1 + the GST rate. If you need only the GST portion of an inclusive total, multiply it by the rate ÷ (1 + the rate)—not by the headline rate.
Choose the right formula for the price you have
Write the applicable GST rate as a decimal and call it r. For example, 10% becomes 0.10 and 9% becomes 0.09. Let the starting amount be the price before GST or the final price including GST, as appropriate.
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| What you know | What you want | Calculation |
|---|---|---|
| GST-exclusive price | GST amount | Exclusive price × r |
| GST-exclusive price | GST-inclusive total | Exclusive price × (1 + r) |
| GST-inclusive total | GST-exclusive price | Inclusive total ÷ (1 + r) |
| GST-inclusive total | GST portion | Inclusive total × r ÷ (1 + r) |
How to add GST to an exclusive price
Multiply the exclusive price by the rate to find the GST amount, then add that tax to the original price. Equivalently, multiply the exclusive price by 1 + the rate to get the inclusive total directly.
For example, the Australian Government’s GST learning resource uses a 10% rate in this worked example: a $160 exclusive amount has $16 GST, for a $176 inclusive total. See the Australian Government GST learning resource.
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How to remove GST from an inclusive price
Divide the inclusive price by 1 + the rate to recover the price before GST. Using the Australian Government example, $176 ÷ 1.10 = $160. The GST included in the $176 total is therefore $16.
How to calculate only the GST included in a total
Use the tax fraction r ÷ (1 + r) on the inclusive total. The fraction is smaller than the headline rate because the tax is already part of the total. At a 10% rate, the fraction is 10/110, or 1/11; at 9%, it is 9/109.
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For a Singapore example, the Inland Revenue Authority of Singapore (IRAS) shows that an inclusive price of $109 at 9% contains $9 GST. The calculation is $109 × 9/109 = $9. See IRAS guidance on computing output tax.
Use the rate that applies to the transaction
The arithmetic is consistent, but the correct rate depends on the country and the particular supply. The official examples here illustrate different rates; they are not interchangeable or a statement of the rate that applies to every transaction:
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- The Australian Government learning resource uses 10% in its example; the page does not state a publication year. Australian Government GST learning resource.
- IRAS’s 2026 example uses 9% and a 9/109 tax fraction. IRAS computing output tax.
- The Guyana Revenue Authority’s 2026 policy gives a 14% VAT rate and the corresponding 7/57 tax fraction. This is a VAT example showing the same mathematics, not guidance on GST law. Guyana Revenue Authority VAT guidance.
Confirm the applicable rate for the country, supply and transaction before doing the calculation.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Rounding and displayed prices are separate questions
Round the result according to the relevant jurisdiction’s invoice, tax or accounting rules. The official sources cited here do not establish one rounding convention that applies everywhere.
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A correct conversion also does not determine whether a seller can advertise or quote a price before GST. Price-display requirements are separate local rules. In Singapore, IRAS says GST-registered businesses must show public prices inclusive of GST, subject to stated exceptions, including qualifying hotels and food-and-beverage establishments that impose a service charge. IRAS states: “You must show the final price customers pay, including GST.” Read IRAS guidance on displaying and quoting prices. This Singapore rule should not be assumed to apply elsewhere.
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Quick Recap
Best Value
- 1. GST Calculator is a handy app which allows you to calculate all 4 GST tax rates at your finger tips. It takes-in an amount (either gross or net) and allows you to add or reduce GST from given amount.
Product prices and availability are accurate as of the date/time indicated and are subject to change. Any price and availability information displayed on Amazon at the time of purchase will apply.
