Reverse GST Calculator

This reverse GST calculator extracts the 15% GST content from any GST-inclusive total in New Zealand. It uses the IRD-prescribed 3/23 fraction method, which is mathematically exact and avoids the rounding errors that can creep in when you simply divide by 1.15 across many transactions. Enter your GST-inclusive total below to see the GST component and the GST-exclusive amount instantly. All calculations use the current 15% GST rate (unchanged since 1 October 2010) and apply to the 2026/27 tax year.

Need to add GST to a price instead? Use our NZ GST Calculator to add 15%.

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.