GST Calculator

Enter the amount, the GST rate and whether the figure is before GST or already inclusive. The calculator uses the rate you enter and does not choose a country, place of supply or rate table.

Rules differ by country. This tool performs the calculation you ask for. What you are entitled to charge, and how it must be presented, is set by your contract and your local law — check both before relying on a figure here.

What this tool does

GST arithmetic has the same direction problem as VAT: if the price already includes tax, multiplying the full price by the rate overstates the tax. The calculator derives the amount before GST from the inclusive total, then reconciles the GST amount and final invoice total.

This is deliberately not a jurisdiction classifier. It does not infer province, state, place of supply, product category or current legal rate. Use the rate that applies to your transaction and let the tool handle the inclusive or exclusive calculation.

How it works

  1. Enter the amount

    Use the amount you need to calculate from. If the figure already includes GST, switch the basis and the amount before GST is derived from it instead of being assumed.

  2. Enter the GST rate

    The rate is supplied by you. Presets are shortcuts only, not a lookup table and not a statement about which rate applies.

  3. Choose inclusive or exclusive

    Exclusive means GST is added on top of the amount. Inclusive means the total already contains GST, so the pre-GST amount and GST amount are extracted from the total.

  4. Check the reconciliation

    The result shows amount before GST, GST amount and invoice total together so the direction of the calculation is visible.

Worked example

A supplier bills $1,000 before GST and enters a 10% GST rate.

Inputs

Amount
$1,000.00
GST rate
10%
Amount entered is
Before GST

Result

Amount before GST
$1,000.00
GST
$100.00
Invoice total
$1,100.00

Why it matters: If the same $1,100 is entered as an inclusive amount, the calculator works backwards to $1,000 before GST and $100 GST instead of adding another $110 on top.

Best practices

  • Use the rate that applies to the transaction; the calculator does not choose it for you.
  • Mark clearly whether the entered amount is before GST or already inclusive.
  • Use inclusive mode for agreed all-in prices so tax is extracted rather than added again.
  • Keep the rate source with your records if you need to justify the rate later.
  • Review local invoice wording and registration-number requirements separately from this arithmetic.

Common mistakes

Adding GST to a price that already includes it

An inclusive price already contains GST. Multiplying the full total by the rate adds tax a second time and overstates the invoice.

Treating a preset as a rate lookup

Preset buttons are just shortcuts into the rate field. They do not determine which rate applies to the sale.

Treating an all-inclusive quote as the taxable value

If a client agreed $1,100 inclusive at 10%, the amount before GST is $1,000 and the GST is $100. Entering $1,100 as the pre-GST amount bills $1,210.

Frequently asked questions

Does this GST calculator choose the rate for me?

No. Enter the rate that applies to your transaction. The calculator only applies that rate to the amount you enter.

How do I calculate GST on an inclusive price?

Divide by 1 plus the rate. $1,100 at 10% contains $100 of GST on an amount before GST of $1,000. Multiplying the inclusive figure by 10% instead gives $110, which overstates the GST.

Can I use this for GST in different countries?

Yes, for the arithmetic. It does not apply country-specific rate tables, registration rules, filing categories or invoice wording.

Does this split GST into local components?

No. It returns one GST amount from the rate you enter. If your invoice needs component labels, add those from your own records or local rules outside this calculator.

Is the amount before or after a discount?

Use the amount your own invoice logic treats as taxable. This calculator does not decide discount, credit-note or adjustment rules.

Turn the answer into an invoice

The result above transfers straight into the invoice generator — dates, amounts and terms already filled in. Free, no account needed.

Open the invoice generator

Sources & further reading

Related

Glossary

Invoice types

Published · General information, not legal, tax or financial advice.