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To add GST to a pre-tax price, multiply the price by 1 + the GST rate. To find GST already included in a total, multiply that total by rate ÷ (1 + rate). For a rate-change comparison, keep the pre-tax price constant and calculate the old and new totals separately. The applicable rate and date depend on the country, location, and type of sale; there is no universal GST rate.
Calculate GST from a pre-tax price
Write the rate as a decimal: for example, 14% becomes 0.14. If P is the pre-tax price and r is the rate:
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- GST amount:
P × r - Price including GST:
P × (1 + r)
For example, at 14%, a pre-tax price of $100 has $14 GST and a $114 tax-inclusive total. This is an arithmetic example, not a statement that 14% applies to your purchase. HM Revenue & Customs illustrates the same method with UK VAT: multiply an exclusive price by 1.2 at 20%, or by 1.05 at 5%. HMRC’s VAT guidance gives those examples.
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Find GST included in a displayed total
If the displayed price T already includes tax, do not multiply the total by the headline rate to extract the tax. The tax is a fraction of the inclusive total:
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- Pre-tax price:
T ÷ (1 + r) - GST included:
T × r ÷ (1 + r), orT − T ÷ (1 + r)
For example, if a tax-inclusive total is $114 and the applicable rate is 14%, the pre-tax amount is $114 ÷ 1.14 = $100, and the included tax is $14. The Guyana Revenue Authority expresses the tax fraction as R/(1+R); at 14%, that fraction is 7/57. Guyana Revenue Authority calculation guidance explains the inclusive-price method.
Compare prices before and after a rate change
For a rate-only comparison, use the same pre-tax base both times. If the old rate is rold and the new rate is rnew:
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- Old total:
P × (1 + r_old) - New total:
P × (1 + r_new) - Difference due to the rate change:
P × (r_new − r_old)
Suppose the pre-tax price is $100 and a rate changes from 10% to 12%. The old total is $110, the new total is $112, and the rate change accounts for a $2 increase. This comparison isolates the effect of the rate; it is a calculation method, not a legal rule. If the seller also changes the pre-tax price, calculate that change separately rather than attributing the entire difference to GST.
Check which rate and date apply
The arithmetic cannot tell you which rate a real transaction should use. Confirm the supply’s tax category, place of supply, and the rules in force for the relevant transaction date. For example, the Canada Revenue Agency says the GST/HST rate depends on place and type of supply; its current table lists Nova Scotia HST at 14% from April 1, 2025. Where PST applies, it also instructs businesses to calculate GST on the price before PST. CRA rate guidance provides the location-specific details.
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Effective-date rules can be more detailed than the date a customer placed an order. In the UK, HMRC says rate changes must be applied immediately from the date they change, and its guidance distinguishes standard-, reduced-, and zero-rated supplies. HMRC’s rate-change guidance describes the UK rule. South Africa provides a separate example: the South African Revenue Service FAQ describes VAT at 15.5% for supplies from May 1, 2025, with a 15.5/115.5 fraction for extracting VAT from an inclusive amount. It says goods entered into before that date but delivered or made available for collection on or after it use the rate on delivery or availability; services use the rate on performance or rendering. SARS’s VAT rate-change FAQ sets out those South African transition details. These examples are not rules for other jurisdictions.
A rate change also does not, by itself, establish that a seller must change a customer’s displayed or contracted total. If an inclusive total is held fixed, changing the rate changes the tax and pre-tax portions represented by that total; whether a seller can or must alter the customer-facing price depends on local law and the contract.
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Round the result appropriately
Keep full precision during intermediate calculations, then round the payable amount where the applicable rules or transaction system require it. Rounding practices are not universal. The Canada Revenue Agency’s online forward and reverse GST/HST calculator accepts either a pre-tax amount or a total after taxes, but warns that its estimates may differ from final totals by a cent or two because of rounding. CRA’s calculator guidance explains the estimate and rounding caveat. For an invoice or legally exact amount, follow the relevant tax authority’s calculation and rounding instructions.
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.
