GST calculator
Add 15% GST, or pull it back out of a total that already includes it.
Source: Inland Revenue · GST 15% since 1 Oct 2010 · Checked Oct 2026Using New Zealand GST of 15%
Continue with these numbers
Sole trader expense log
Take the GST this work leaves you to account for into the expense log, with each entry split at the published rate.
Carries $150.00 GST
How it works
GST in New Zealand is 15% of the price before tax. Adding it is simple multiplication. Taking it back out of a total is where people go wrong, because 15% of the GST-inclusive figure is not the GST — the tax was calculated on the smaller number.
If you are GST registered, the GST you charge is not your money. You collect it and pass it on, which is why invoices show it as its own line rather than folding it into the price.
The formula
Two directions, two different sums. The second is the one worth remembering.
add: × 1.15 · remove: ÷ 23 × 3
+To add GST, multiply the pre-tax amount by 1.15
−To find the GST inside a total, divide by 23 and multiply by 3
=Dividing the total by 1.15 gives you the pre-tax amount
A worked example
A job quoted at $1,000 before GST. Multiply by 1.15 and the invoice total is $1,150, of which $150.00 is GST.
Common questions
Why divide by 23 and multiply by 3?
Because a GST-inclusive total is 115% of the price, and GST is 15 of those 115 parts. Fifteen over one hundred and fifteen simplifies to three over twenty-three.
Do I have to register for GST?
Registration is compulsory once your turnover passes $60,000 in any twelve-month period. Below that it is optional, and worth doing only if your customers are themselves registered.
Can I claim the GST on business purchases?
If you are registered, yes, on purchases made for taxable supplies. You claim the GST portion, not the whole amount, which is what this calculator gives you.
Which supplies are not 15%?
Exports and some financial services are zero-rated, and residential rent and donated goods are exempt. This calculator handles standard-rated supplies only.
The rules behind this calculator
Written up, with the page each rule came from.
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