GST Calculator
Add GST to a net price, or work out the GST hidden inside a gross price.
Goods and Services Tax works in two directions, and it's easy to reach for the wrong one. Sometimes you have a price before tax and need to add GST; other times you have a tax-inclusive total and need to pull the GST back out. This calculator does both. Pick the mode, enter the amount and the GST rate, and it shows the tax portion along with the net and gross figures. Everything is worked out in your browser.
Adding versus extracting GST
The two modes use different arithmetic, and this is the part people most often get wrong:
Add: GST = Net × R ÷ 100 • Extract: Net = Gross ÷ (1 + R ÷ 100)
To add GST you multiply the net price by the rate. To extract it you can't just take that percentage off the gross — you divide by one plus the rate. On a $110 total that already includes 10% GST, the net is $110 ÷ 1.10 = $100 and the GST is $10. Taking 10% off $110 would wrongly give $99, which is why the divide-by method matters for inclusive prices.
Choosing the right rate
GST rates differ by country and sometimes by product category, and some regions use VAT or a combined sales tax instead — the maths is identical, only the name and rate change. Enter whatever rate applies to your situation rather than assuming a default. This tool handles the calculation for any rate you type; confirming the correct rate for a specific good or jurisdiction is up to you, and for formal invoicing you should follow your local tax authority's rounding rules.
Frequently Asked Questions
Why can't I just subtract the rate to remove GST?
Because the GST is a percentage of the net price, not the gross. On a tax-inclusive total you divide by one plus the rate — dividing $110 by 1.10 gives $100 net, whereas simply taking 10% off $110 would incorrectly give $99.
What's the difference between the two modes?
Add mode starts from a price before tax and adds GST on top. Extract mode starts from a price that already includes GST and separates out the tax and the net amount.
Does it work for VAT or sales tax too?
Yes. The calculation is the same for any percentage-based consumption tax — just enter the rate that applies where you are.
Is my data stored?
No. The calculation runs in your browser and nothing you enter is uploaded or saved.