Reverse GST Calculator

This calculator works out how much GST is hidden inside a total price, and what that price would be without it. If you have a GST-inclusive figure, such as an invoice, a receipt or a quoted price, and need to split it into the GST-exclusive amount and the GST itself, this does the maths instantly using the method Inland Revenue prescribes, rather than a rough estimate. You enter the GST-inclusive total in the amount field, and the GST percentage defaults to the current 15% rate but can be changed if you are checking an older transaction. The calculator multiplies your total by 3 and divides by 23, the 3/23 fraction, mathematically identical to dividing by 1.15, and returns two figures: the GST-Exclusive Amount and the GST Content, the dollar value of GST buried in your total. This is the reverse of adding GST on, and it is not the same as subtracting 15% from the total, which gives the wrong answer because the percentage ends up applied to the wrong base. Use it to check supplier invoices, reconcile receipts for a GST return, work out the true cost of something before GST, or verify a quote. For a transaction before October 2010, change the percentage field to 12.5% or 10% to match the rate that applied then. To add GST to a price instead, use our GST Calculator.

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The 3/23 Method Explained

The 3/23 fraction is the official Inland Revenue method for extracting GST from a GST-inclusive total. It comes from a simple piece of algebra:

• If the GST-exclusive amount is x, then the GST-inclusive total is x + 15% of x = 1.15x.
• The GST component is 0.15x.
• As a proportion of the GST-inclusive total, GST = 0.15x / 1.15x = 15/115 = 3/23.

GST content = GST-inclusive total × 3 ÷ 23

The reason IRD prefers the fraction over "divide by 1.15" is rounding. Fifteen divided by 115 equals 0.13043478260869565..., a recurring decimal. The fraction 3/23 is exact. For a single invoice it makes no practical difference; across hundreds of invoices in a GST return, using the exact fraction keeps your totals consistent.

Worked Examples

Example 1: A $575 invoice

• GST-inclusive total: $575
• GST content: $575 × 3 ÷ 23 = $75
• GST-exclusive amount: $575 − $75 = $500
• Verification: $500 × 1.15 = $575 ✓

Example 2: A $2,300 tradie invoice

• GST-inclusive total: $2,300
• GST content: $2,300 × 3 ÷ 23 = $300
• GST-exclusive amount: $2,300 − $300 = $2,000

Example 3: A $1,150 fuel receipt

• GST-inclusive total: $1,150
• GST content: $1,150 × 3 ÷ 23 = $150
• GST-exclusive amount: $1,150 − $150 = $1,000

Example 4: A messy number, $847.50

• GST-inclusive total: $847.50
• GST content: $847.50 × 3 ÷ 23 = $110.54 (rounded)
• GST-exclusive amount: $847.50 − $110.54 = $736.96

Why Subtracting 15% Gives the Wrong Answer

One of the most common GST mistakes is assuming that if adding 15% takes you from $100 to $115, then subtracting 15% from $115 should take you back to $100. It does not.

$115 minus 15% means $115 × 0.85 = $97.75, not $100. The reason is that the 15% is applied to a different base each way: when adding, the base is $100; when subtracting, the base is $115. The correct reverse calculation always uses the 3/23 fraction, or equivalently, divides by 1.15.

Historical Rates

New Zealand's GST rate has changed twice since the tax was introduced:

• 1 October 1986 – 30 June 1989: 10% (fraction 1/11)
• 1 July 1989 – 30 September 2010: 12.5% (fraction 1/9)
• 1 October 2010 – present: 15% (fraction 3/23)

For historical transactions, change the percentage field above to the relevant rate.

Frequently Asked Questions

What is the reverse GST formula in New Zealand?

Multiply the GST-inclusive total by 3 and divide by 23. That gives the GST content. Subtract it from the total to get the GST-exclusive amount.

Why does subtracting 15% not work?

Because the percentage is applied to different bases. Adding 15% to $100 gives $115; subtracting 15% from $115 gives $97.75, not $100. Always divide by 1.15 or use the 3/23 fraction.

How do I remove GST from $1,150?

$1,150 × 3 ÷ 23 = $150 GST. The GST-exclusive amount is $1,150 − $150 = $1,000.

Is the 3/23 method required, or can I just divide by 1.15?

Either works and gives the same result. IRD prefers the fraction for rounding consistency in large-volume returns. For single invoices, either method is fine.

What is the GST fraction for 12.5%?

1/9. Multiply the GST-inclusive total by 1 and divide by 9. For example, $112.50 × 1 ÷ 9 = $12.50 GST.

Related Calculators

• NZ GST Calculator: add 15% GST to a GST-exclusive price.
• GST on Imports: the Customs formula for imported goods.
• GST Due Dates 2026/27: filing frequency and deadlines.
• GST Registration Threshold: the $60,000 rule explained.
• Business GST Calculator: net GST payable across multiple lines.
• PAYE Calculator: take-home pay after income tax.