GST calculator NZ: add or remove 15%

How the two directions work

Adding GST is simple multiplication. If a supplier quotes $240.00 plus GST, the tax is 15% of that, $36.00, and the customer pays $276.00. The calculator does exactly this, then rounds to the nearest cent.

Removing GST is where people slip. The GST inside an inclusive price is not 15% of that price, because the 15% was charged on the smaller, pre-GST amount. The share that is tax works out to 3/23 of the inclusive price. Inland Revenue uses the same fraction in its own guidance. So a receipt for $89.99 contains $11.74 of GST, and the price before GST was $78.25. Dividing the inclusive price by 1.15 gives the same answer from the other side.

Worked examples

These figures come from the same code that runs the calculator above.

Now a small invented invoice. A café orders three boxes of takeaway cups at $19.99 each, GST included. Working the GST out once on the $59.97 total gives $7.82, which is the figure the calculator shows. Rounding each line separately gives $2.61 per box and $7.83 altogether. Neither is wrong; the one-cent gap comes purely from rounding three times instead of once. When you enter several lines, the calculator lists each one and points out any gap like this, so you can match whatever your accounting software does.

Mistakes that cost money

• Taking 15% off an inclusive price. Knocking 15% off $115.00 leaves $97.75, but the real pre-GST price is $100.00. The error grows with the amount, and on a large invoice it adds up to real money.
• Mixing inclusive and exclusive figures. A quote that lists some items "plus GST" and others "incl. GST" needs each line handled on its own basis before you total it. Convert everything to one basis first.
• Assuming every sale carries GST. Some supplies are zero-rated, meaning GST is charged at a rate of zero; certain services supplied to people outside New Zealand are one example. Others are exempt altogether, such as most financial services and donated goods sold by a non-profit. Inland Revenue lists these on its pages about Zero-rated supplies and Exempt supplies. This calculator always uses the standard rate, so don't use it for those supplies.
• Adding GST when you aren't registered. Inland Revenue says you must register if your taxable turnover reached $60,000 in the last 12 months or you expect it to in the next 12 months. Adding GST to your prices also means you have to register. If you are under the threshold and not registered, you don't charge GST at all.

Limits of this calculator

It handles the everyday case: one standard rate applied to ordinary goods and services. It does not deal with land transactions, second-hand goods, imported goods, or the special rules some businesses use. It also can't tell you what records you need to keep, though it helps to know that a GST-registered buyer can ask for taxable supply information on supplies over $200. For anything beyond simple arithmetic, check Inland Revenue's guidance or ask an accountant.

What you type stays in your browser. Amounts and descriptions are never sent anywhere or saved. Only the add-or-remove choice is remembered, in the page address after the # sign, so a bookmark opens the calculator in the same mode.

Working out pay rather than prices? The take-home pay calculator covers PAYE, ACC and KiwiSaver. To understand the tax itself, read the guide to adding and removing GST.

Last reviewed: 2 October 2026