How to calculate GST from a total price

Understand how GST inclusive and exclusive amounts work before calculating tax values.

Calculators4 min read

GST can be added or extracted

When a price is GST exclusive, you add GST on top of the base amount. When a price is GST inclusive, the tax is already inside the total.

The calculation is different depending on which number you start with.

If an item costs 100 before GST and the GST rate is 10%, the GST amount is 10 and the total is 110.

This is the common add-on calculation used for quotes or pre-tax prices.

If the final price is 110 and GST is 10%, the GST portion is not 11. The tax is extracted from the total by dividing by 1.10 first.

A calculator helps avoid that common mistake when working from receipt totals.

Is GST calculated on the before-tax amount?

When adding GST, yes. When extracting GST from a final price, the tax is calculated from the included total.

Can the GST rate change?

Yes. Always use the tax rate that applies to your country, state, product, or invoice.