GST Calculator NZ: How to Add, Remove and Work Out 15% GST

Short answer: Every NZ GST calculator runs one of three sums. To add GST, multiply the GST-exclusive price by 1.15. To find the GST inside a GST-inclusive total, multiply the total by 3 and divide by 23. To remove GST, divide the total by 1.15. GST in New Zealand is 15%, so $100 plus GST is $115, and a $115 total contains $15 of GST. The most common mistake is taking 15% of a total that already includes GST.

GST is charged at 15% on most goods and services in New Zealand, according to Inland Revenue’s GST page (checked October 2026, in the 2027 tax year, 1 April 2026 – 31 March 2027). The formulas below hold for as long as the rate stays at 15%. This page is for sole traders and small businesses who need to price jobs, check invoices and prepare a GST return. You only charge GST once registered. Registration is compulsory if your turnover was at least $60,000 in the last 12 months or you expect it to reach $60,000 in the next 12, and Inland Revenue’s registering for GST page gives you 21 days to register once that applies. Self-employed taxes in NZ puts GST alongside your other obligations, and how to invoice as a contractor in NZ shows how GST has to appear on what you send. The New Zealand guides hub lists the rest of the series.

What are the formulas for calculating GST in NZ?

There are three, and you only need to know which number you are starting from: a price before GST (GST-exclusive) or a price that already includes it (GST-inclusive).

You have	You want	Formula	$100 / $115 example
GST-exclusive price	GST-inclusive price	price × 1.15	$100 × 1.15 = $115
GST-exclusive price	GST amount	price × 0.15	$100 × 0.15 = $15
GST-inclusive total	GST amount	total × 3 ÷ 23	$115 × 3 ÷ 23 = $15
GST-inclusive total	GST-exclusive price	total ÷ 1.15	$115 ÷ 1.15 = $100

That is the whole of GST arithmetic. There is no interactive calculator on this page; the formulas are more useful than one, because they work on your phone’s calculator, in a spreadsheet and in your head.

How do I add GST to a price?

Multiply the GST-exclusive price by 1.15. That adds the 15% GST and gives you the GST-inclusive price in one step.

If you want to show the GST separately, as most invoices do, do it in two steps. Inland Revenue’s GST guide (IR375) uses this example: an item sold for $100 excluding GST, multiplied by 0.15, gives $15 of GST, for a total of $115.

Tradie example. You quote a bathroom job at $4,800 plus GST. GST is $4,800 × 0.15 = $720, and the client pays $4,800 × 1.15 = $5,520.

How do I work out the GST in a GST-inclusive total?

Multiply the total by 3, then divide by 23. That is Inland Revenue’s own method on its calculating your GST page, where $230 × 3 ÷ 23 = $30.

Why 3/23? GST is 15% of the exclusive price, so a GST-inclusive total is 115% of it. The GST is therefore 15/115 of the total, which simplifies to 3/23, roughly 13.04%.

This is where most errors happen. Taking 15% of a GST-inclusive total overstates the GST. On a $115 receipt, 15% is $17.25, but the GST in it is only $15. If you claim GST on expenses this way, every claim is too high.

Inland Revenue also allows dividing the total by 7.66666666, but says you must use at least eight decimal places or the answer can be wrong. The 3/23 fraction is exact, so use that.

How do I remove GST from a price?

Divide the GST-inclusive total by 1.15 to get the GST-exclusive price. You get the same answer by subtracting the 3/23 figure from the total.

Example. A supplier invoice comes to $1,380 including GST. $1,380 ÷ 1.15 = $1,200 excluding GST, and the GST is $1,380 − $1,200 = $180, which matches $1,380 × 3 ÷ 23.

What you should not do is subtract 15% from the total. $1,380 minus 15% is $1,173, which is $27 short of the real GST-exclusive price.

Worked examples: GST on common NZ amounts

These are all at the standard 15% rate, rounded to the cent.

Starting amount	Calculation	Result
$250 excl GST	× 1.15	$287.50 incl GST (GST $37.50)
$1,200 excl GST	× 1.15	$1,380.00 incl GST (GST $180.00)
$86.95 excl GST	× 1.15	$99.99 incl GST (GST $13.04)
$230 incl GST	× 3 ÷ 23	GST $30.00
$500 incl GST	× 3 ÷ 23	GST $65.22
$49.99 incl GST	× 3 ÷ 23	GST $6.52
$500 incl GST	÷ 1.15	$434.78 excl GST
$5,520 incl GST	÷ 1.15	$4,800.00 excl GST
$49.99 incl GST	÷ 1.15	$43.47 excl GST

Check any line by adding the parts back together: $434.78 + $65.22 = $500.00.

How should I round GST to the cent?

Round the GST amount to the nearest cent, with half a cent or less rounded down. The Goods and Services Tax Act 1985 says a fraction of a cent that is half a cent or less may be disregarded, and a fraction above half a cent counts as a full cent.

In the table above, $86.95 × 0.15 = $13.0425, so the GST is $13.04. And $500 × 3 ÷ 23 = $65.2174, so the GST is $65.22.

Two habits keep rounding from drifting:

• Work out GST on the invoice total, not line by line, unless you show GST per line. Rounding ten lines separately can leave you a few cents off the total.
• Calculate the exclusive price from the GST, not the other way round, when you start from an inclusive total: find the GST with 3/23, round it, then subtract. The parts then always add back to the total.

How do I set up a GST calculator in a spreadsheet?

Put your amount in cell A2 and use these formulas. They work in Excel, Google Sheets and Numbers.

Calculation	Formula
Add GST (inclusive price)	=ROUND(A2*1.15,2)
GST on an exclusive price	=ROUND(A2*0.15,2)
GST inside an inclusive total	=ROUND(A2*3/23,2)
Remove GST (exclusive price)	=A2-ROUND(A2*3/23,2)

The last formula uses the 3/23 figure and subtracts it, so the exclusive price and GST always add back to the original total. A spreadsheet’s ROUND function rounds an exact half cent up, where the GST Act lets you round it down. That only matters when a GST figure ends in exactly half a cent, which is rare.

How does calculating GST feed into my GST return?

Your GST return is the same arithmetic applied to a whole period: the GST on your sales minus the GST on your business purchases. If the result is positive you pay Inland Revenue; if it is negative you get a refund.

Example for a two-month period:

Item	GST-inclusive	GST (× 3 ÷ 23)
Sales invoiced	$23,000	$3,000
Business purchases with GST	$4,600	$600
GST to pay		$2,400

Whether you count sales when you invoice or when you are paid depends on your accounting basis, payments or invoice. Under the invoice basis you include sales you have invoiced and purchases you have been invoiced for and hold taxable supply information for, even if no money has changed hands yet. Purchases only count if they were for your taxable activity and actually included GST.

Not everything is charged at 15%. Some supplies are zero-rated (GST at 0%) and some are exempt, so the formulas above only apply to standard-rated supplies. For the filing steps, see how to file a GST return in NZ, and for when each return is due, the GST due dates calendar.

Keeping GST figures straight, and where does Keel fit?

The formulas are simple; the hard part is having every invoice and receipt in one place when the return is due, with the GST on each one visible.

Keel: Invoice Maker & Receipts, an iPhone app by Ilura Technology OÜ, is a record keeper for that. You create quotes that turn into invoices in one tap, send PDF invoices, capture receipts and expenses, and keep each one under its job and customer. “Who owes you” lists unpaid invoices and prepares reminder drafts that you review and send yourself; nothing is sent automatically. Records stay on your iPhone, with no account, no bank connection and no cloud sync, and the App Store privacy label reads “Data Not Collected”.

Be clear on its limits: Keel does not file your GST return, does not connect to Inland Revenue or myIR, and does not work out what you owe for a period. It keeps the invoices and receipts you or your accountant build the return from. It is free with no invoice limit (free invoices carry a small “Made with Keel” footer); Keel Lifetime is a one-time purchase of $249.99 USD (the App Store shows your local price) that adds custom branding, signature, premium templates and accountant-ready exports and reports. Keel on the App Store.

Frequently asked questions

How do I calculate 15% GST in NZ? If your price does not include GST yet, multiply it by 0.15 to get the GST, or by 1.15 to get the GST-inclusive total. A $200 job plus GST is $30 of GST and $230 in total. If your price already includes GST, do not take 15% of it; multiply by 3 and divide by 23 instead.

How do I work out the GST from a total in NZ? Multiply the GST-inclusive total by 3 and divide by 23. This is the method Inland Revenue uses, and it is exact because GST is 15/115 of an inclusive price, which simplifies to 3/23. For example, a $230 total contains $30 of GST, and a $500 total contains $65.22 once rounded to the cent.

How do I remove GST from a price in NZ? Divide the GST-inclusive price by 1.15. A $1,380 total becomes $1,200 excluding GST, with $180 of GST. Do not subtract 15% from the total, because that removes too much: $1,380 minus 15% is $1,173, not $1,200. You can also find the GST with 3/23 and subtract it from the total.

Why is GST 3/23 and not 15% of the total? Because the 15% is charged on the GST-exclusive price, not the total. A GST-inclusive total is 115% of the exclusive price, so the GST share is 15 out of 115, which reduces to 3/23, or about 13.04%. Taking 15% of an inclusive total overstates the GST by roughly two percentage points.

What does GST exclusive mean? GST exclusive, often written “excl GST” or ”+ GST”, means the price before GST is added. GST inclusive means the price already contains GST. A quote of $1,000 + GST costs the client $1,150, while a quote of $1,000 incl GST contains $130.43 of GST and $869.57 for the work itself. Always say which one your quote is.

What method does Inland Revenue use to calculate GST? Inland Revenue’s guidance sets out two sums: multiply a GST-exclusive price by 15% to work out the GST to add, and multiply a GST-inclusive price by 3 and divide by 23 to find the GST inside it. Any GST calculator for New Zealand, online or in accounting software, is doing one of these, so you can always check its answer by hand.

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This article is general information, not tax advice. Consult a qualified New Zealand tax professional.