Adding GST to a price
To add GST, multiply the net price by the GST rate and divide by 100. Add the tax amount to the net price to get the tax-inclusive total. For a net price of 1,000 at 18%, the GST is 180 and the gross total is 1,180.
Extracting GST from an inclusive total
Do not simply take 18% of a price that already includes 18% GST. The tax component is based on the smaller net value. Divide the gross price by 1 plus the tax rate expressed as a decimal to recover the net amount, then subtract the net amount from the gross total.
For a gross total of 1,180 at 18%, the net amount is 1,000 and the GST component is 180.
Check the applicable rate
Rates can depend on the product, service, place, transaction date, registration status and current rules. A calculator cannot determine the correct legal classification. Confirm the applicable rate with current official guidance or a tax professional.
Keep a clear record
Show the net amount, rate, tax amount and gross amount separately. Retain source invoices and use consistent rounding. Small differences can occur when tax is rounded per line instead of on an invoice total.
The GST calculator performs arithmetic using the rate you enter. It does not decide whether GST applies or replace accounting or tax advice.