GST basics
GST calculator: understand tax-added and tax-included prices
Use the right calculation for a taxable amount versus a total that already includes the GST rate you entered.
Updated July 21, 2026 · 4 min read
Start by checking which price you have
A taxable amount is a base price before GST is added. A tax-included amount is the final total where the GST portion is already inside the number. The calculation changes depending on which price you start with.
For a base amount, multiply by the entered rate to find the GST amount, then add it. For an inclusive total, first separate the taxable value using the entered rate, then find the difference.
The rate must be verified separately
This calculator accepts the GST rate you enter, but it cannot determine the rate or tax treatment for a particular supply. Classification, exemptions, place of supply, and other facts can affect a real transaction.
Use the relevant invoice and official GST resources to check the rate and transaction details. Seek qualified advice before filing, invoicing, or making a decision where GST treatment matters.
A calculation is not an invoice
A tax invoice has information beyond a base amount and rate. Official guidance covers items such as value, rate, tax amount, and other transaction details.
Use this tool for a quick estimate or a price comparison. Keep the supplier's actual invoice as the record for a purchase or business transaction.
Sources and further reading