To calculate a tax-inclusive price, multiply the pre-tax price by 1 plus the applicable GST rate. To find GST already included in a displayed total, multiply that total by r ÷ (1 + r), where r is the rate as a decimal. For a rate-only comparison, keep the pre-tax price constant and calculate the total once at each rate. The arithmetic is general; the applicable rate, tax category and effective date depend on the country and transaction.
Calculate GST from a pre-tax price
Write the applicable rate as a decimal: for example, 14% becomes 0.14. If P is the price before tax and r is the rate:
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- GST amount: P × r
- Price including GST: P × (1 + r)
For example, at a hypothetical applicable rate of 14%, a pre-tax price of 100 becomes 114 including tax, and the tax amount is 14. Use the rate that actually applies to the supply and location; 14% here is an illustration, not a universal GST rate.
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HM Revenue & Customs illustrates the same multiplier with UK VAT: at the 20% standard rate, multiply the pre-tax amount by 1.2; at 5%, multiply by 1.05. These are examples for the UK rates stated in its VAT guidance, not GST rates for every country.
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Work out GST included in a displayed price
If the displayed total T already includes tax, divide by 1 plus the rate to recover the pre-tax amount. Then subtract that amount from the total to find the tax:
- Pre-tax amount: T ÷ (1 + r)
- GST included: T × r ÷ (1 + r), or T − T ÷ (1 + r)
At 14%, a tax-inclusive total of 114 contains 14 in tax: 114 ÷ 1.14 = 100 before tax, and 114 − 100 = 14. Do not simply multiply an inclusive total by 14%; that would calculate 14% of the total, not the tax portion embedded in it. The Guyana Revenue Authority expresses the inclusive tax fraction as R ÷ (1 + R); for its 14% example, that fraction is 7/57. See its VAT calculation guidance.
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Compare the final price before and after a rate change
To isolate the mathematical effect of a rate change, use the same pre-tax base price for both calculations. If the old rate is r₁ and the new rate is r₂:
- Old total: P × (1 + r₁)
- New total: P × (1 + r₂)
- Difference due to the rate alone: P × (r₂ − r₁)
For example, if a pre-tax price of 100 is taxed first at 10% and then at 12%, the totals are 110 and 112. The rate-only increase is 2. This comparison is a calculation method, not a rule requiring a seller to change a customer-facing price.
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If the seller also changes the underlying price, the difference between the two totals reflects both changes. Calculate each total from its own pre-tax base if you want to compare what customers actually pay, and keep that comparison separate from the rate-only calculation.
What changes if a price is already tax-inclusive?
A tax-inclusive total can be treated in two different ways. If the seller keeps the displayed total fixed, a new rate changes how much of that total represents tax and how much represents the pre-tax amount. If the seller keeps the pre-tax base fixed instead, the tax-inclusive total changes with the rate.
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The formula alone cannot establish whether an advertised or contracted price must or may change. That depends on local law and the contract. Check the price terms as well as the tax calculation when a rate changes.
Check the jurisdiction, supply and effective date
There is no single GST rate or transition rule that applies everywhere. Before using a rate, identify the country or tax jurisdiction, place of supply, type of product or service, and whether that supply is standard-rated, reduced-rated, zero-rated or otherwise treated specially. Then check which date determines the rate for the transaction.
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- United Kingdom: HM Revenue & Customs says rate changes must be applied immediately from the date they change. Its guidance distinguishes standard, reduced and zero-rated supplies. See UK VAT guidance.
- Canada: The Canada Revenue Agency says the GST/HST rate depends on the place and type of supply. Its rate table lists Nova Scotia HST at 14% from April 1, 2025. Where provincial sales tax (PST) applies, the CRA also says to calculate GST on the price before PST. See the GST/HST rate guidance.
- South Africa: The South African Revenue Service describes VAT at 15.5% for supplies from May 1, 2025. Its FAQ gives 15.5/115.5 as the fraction for extracting VAT from an inclusive amount. It also describes transition rules for transactions entered into before May 1 but delivered or performed on or after that date: for goods, the relevant date is delivery or availability for collection; for services, it is performance or rendering. These are South African rules, not a general GST rule. See the SARS VAT rate-change FAQ.
These examples show why an old and new rate cannot be selected by date alone without checking the applicable jurisdiction and supply. For an actual transaction, consult the relevant tax authority’s current rules.
Round the result and verify the invoice amount
Keep precision through intermediate calculations and round the payable amount at the point required by local rules or the invoicing system. The Canada Revenue Agency’s forward and reverse GST/HST calculator accepts either a pre-tax amount or a total after taxes, but warns that its estimates can differ from final totals by a cent or two because of rounding. Use the tax authority’s invoicing and rounding guidance when you need an exact legal amount. The CRA calculator is at Calculating GST/HST; the guidance does not establish one rounding convention for every jurisdiction.
Quick Recap
Quick check before using a rate
- Confirm whether the starting figure is tax-exclusive or tax-inclusive.
- Use the rate and tax category that apply to the supply, not a rate assumed from another country or product.
- For a rate-only comparison, hold the pre-tax base constant; account separately for any price or contract change.
- Check the applicable place-of-supply and transition-date rules.
- Follow local rounding and invoice requirements for the final payable 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.
