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 rate. If you need just the GST portion of an inclusive total, multiply it by rate ÷ (1 + rate)—not by the headline rate.
Use the right formula for the price you have
Write the applicable GST rate as a decimal: for example, 10% becomes 0.10. Let r represent that rate.
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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 amount included in the total | Inclusive total × r ÷ (1 + r) |
How to add GST to an exclusive price
Multiply the exclusive price by the rate to find the GST, or multiply it by 1 plus the rate to get the final inclusive price.
For example, the Australian Government’s GST learning resource uses a 10% example: a $160 exclusive price has $16 GST, making the inclusive total $176. The rate in this worked example is not a statement that 10% applies to every transaction or jurisdiction. Australian Government GST learning resource.
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How to remove GST from an inclusive price
Divide the inclusive total by 1 plus the rate to recover the exclusive price. Subtract that result from the total if you need the GST amount.
Using the $176 total in the Australian Government example, $176 ÷ 1.10 = $160 before GST. The difference, $16, is the GST.
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Extract the GST directly
To find only the GST inside an inclusive total, use the tax fraction r ÷ (1 + r). At a 10% rate, that fraction is 10/110, or 1/11—not 10% of the inclusive total.
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Examples at other rates
The calculation stays the same when the rate changes, but the applicable rate depends on the jurisdiction and transaction. The following are official examples, not universal rates for all sales in those places.
| Source and example | Inclusive total | Tax fraction | Tax included |
|---|---|---|---|
| Inland Revenue Authority of Singapore, 2026: 9% example | $109 | 9/109 | $9 |
| Guyana Revenue Authority, 2026: 14% VAT policy example | Not stated as an inclusive amount by the cited policy | 7/57 | Not stated as an amount by the cited policy |
Singapore’s example shows why the tax fraction is not the same as the headline rate: $109 × 9/109 = $9 GST. Guyana’s Revenue Authority discusses VAT rather than GST; its 14% rate and 7/57 tax fraction illustrate the same arithmetic, not a claim about GST law. IRAS GST rate-change guidance; Guyana Revenue Authority VAT guidance.
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Check the rate and rounding before using the result
- Confirm the rate that applies to the country, supply, and transaction; do not assume the examples above apply to your sale.
- Convert the percentage to a decimal before using the formulas: 9% is 0.09, for instance.
- Use the tax fraction to extract GST from an inclusive total rather than multiplying that total by the headline rate.
- Round according to the applicable jurisdiction’s invoice, tax, or accounting rules. There is no universal rounding convention established here.
Price display is a separate legal question
Calculating a GST-inclusive price does not determine whether a seller may advertise or quote an exclusive price. Display requirements depend on local law. In Singapore, the Inland Revenue Authority of Singapore says, “You must show the final price customers pay, including GST.” Its guidance also describes exceptions, including qualifying hotels and food-and-beverage establishments that impose a service charge. This is Singapore-specific guidance, not a universal display rule. IRAS guidance on displaying and pricing GST.
IRAS also gives a Singapore example in which a customer bargains an inclusive $109 price down to $90: the GST to account for is 9/109 of the final agreed $90. The calculation uses the final price, not the original advertised total. IRAS GST rate-change guidance.
Quick Recap
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- 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.
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