Reverse GST calculator
A reverse GST calculator takes 15% GST back out of a New Zealand price that already includes it. Type the GST inclusive total. The GST is that total times 3 divided by 23, and what is left is the price before GST.
Working paper. The amount is treated as including 15% GST. GST is the total times 3 divided by 23, rounded to the nearest cent. Zero rated and exempt supplies are not split this way.
Checked 29 October 2026 against GST guide IR375. Free working paper, not run by Inland Revenue. Nothing you type is stored.
How reverse GST works
A New Zealand price that includes 15% GST splits into 23 equal parts. Twenty parts are the price before GST and three parts are GST. So the GST is 3/23 of the total, about 13.04%, and the price before GST is 20/23 of it. A $115 total holds $15 of GST and $100 before GST.
One $115 total, cut into 23 parts of $5. Three parts are GST.
The fraction comes straight from the rate. GST is 15% of the price before GST, so every $115 of total holds $15 of GST, and 15/115 reduces to 3/23. GST guide IR375 sets out the same three moves: multiply the GST inclusive price by 3, divide by 23, then subtract that GST to get the GST exclusive amount.
The calculating your GST page runs one pair of headphones through both directions:
This page only runs Arthur's direction. Adding GST to a price that does not include it is on the GST calculator. To use the reverse GST calculator above:
- Type the GST inclusive total from the invoice, receipt, quote, or bank line.
- Read the GST in the price: the total times 3 divided by 23, rounded to the cent.
- Read the price before GST: the total minus that GST.
Three ways to the same GST
Multiplying by 3 and dividing by 23, dividing by 1.15, and dividing by 7.66666666 are one sum written three ways. Done in full, they agree to the cent.
Multiply by 3, divide by 23
$115 times 3 is $345. $345 divided by 23 is $15 of GST. $115 minus $15 is $100.
One calculation, exact every time. This calculator uses it.
Divide by 1.15, then subtract
$115 divided by 1.15 is $100 before GST. $115 minus $100 is $15 of GST.
Two steps. Carry the first answer across with care on a phone calculator.
Divide by 7.66666666
$115 divided by 7.66666666 is $15 of GST.
Keep at least 8 decimal places. Using 7.67 on a $10,000 total gives $1,303.78 instead of $1,304.35.
Multiplying by 3 and then dividing by 23 always gives an accurate answer, so it is the safest sum to run by hand. Why 15/115 reduces to 3/23, and how 20/23 gives the price before GST, is shown line by line in our GST maths guide.
15% of $115 is $17.25. Taking that off leaves $97.75, not the $100 price before GST. The 15% belongs to the price before GST, not to the total. On a $5 cauliflower, the GST is 65 cents and the price before GST is $4.35. A 15% discount would leave $4.25.
Reverse GST on common totals
Each row is worked at 15%: the total times 3 divided by 23 for the GST, then the total minus that GST for the price before GST, rounded to the cent. The last column shows what a flat 15% of the total would wrongly give.
| Total including GST | GST in the total (3/23) | Price before GST | 15% of the total (wrong) |
|---|---|---|---|
| $10.00 | $1.30 | $8.70 | $1.50 |
| $20.00 | $2.61 | $17.39 | $3.00 |
| $50.00 | $6.52 | $43.48 | $7.50 |
| $100.00 | $13.04 | $86.96 | $15.00 |
| $115.00 | $15.00 | $100.00 | $17.25 |
| $230.00 | $30.00 | $200.00 | $34.50 |
| $500.00 | $65.22 | $434.78 | $75.00 |
| $1,000.00 | $130.43 | $869.57 | $150.00 |
| $1,150.00 | $150.00 | $1,000.00 | $172.50 |
| $2,000.00 | $260.87 | $1,739.13 | $300.00 |
| $2,300.00 | $300.00 | $2,000.00 | $345.00 |
| $5,000.00 | $652.17 | $4,347.83 | $750.00 |
| $10,000.00 | $1,304.35 | $8,695.65 | $1,500.00 |
| $23,000.00 | $3,000.00 | $20,000.00 | $3,450.00 |
Reverse GST or reverse charge?
The names sound alike, but they run in opposite directions. Reverse GST takes GST out of a total. The reverse charge makes you add 15% GST to services bought from an overseas supplier.
| Reverse GST (this page) | Reverse charge | |
|---|---|---|
| What it does | Finds the GST already inside a GST inclusive price | Adds GST of 15% to the price of imported services |
| When | Any time you hold a total that includes 15% GST | Services from a non resident supplier, where the intended or actual taxable use is less than 95% of total use |
| The sum | Total times 3 divided by 23 | Price of the services times 0.15 |
| Example | A $460 total holds $60 of GST and $400 before GST | $400 of services from overseas means $60 of GST to account for |
| Who is treated as the supplier | The seller stays the supplier | You are, for registration, output tax, and record keeping |
The reverse charge can apply whether or not you are already registered for GST, and imported services can push turnover over the $60,000 registration threshold. When and how to account for it is set out in our guide to GST on imported services. Reverse GST calculators that split tax into CGST and SGST, or offer 5%, 18%, and 40% rates, are built for India's GST, not New Zealand's.
Totals from before October 2010
The fraction changes with the rate. It is always the rate divided by 100 plus the rate, so an older total needs its own fraction.
This calculator works at 15% only. The rate that applies follows the time of supply, which in most cases is when the supplier issues an invoice or receives payment. A GST return period that straddled 1 October 2010 was split into two parts, one at each rate. The time of supply rules are explained in our GST time of supply guide.
Where the 3/23 sum turns up
Reverse GST is the sum behind input tax claims, bad debts, and online imports. These cards use realistic totals, and each one ends with what to type above.
Use case: Dunedin plumber
Situation: A GST registered plumber pays $1,380 for a van service. The invoice says the total includes GST at the standard rate.
GST: $1,380 times 3 divided by 23 is $180. The service cost $1,200 before GST.
Action: Type 1380. The $180 is the GST in the bill. How much of it can be claimed depends on business use, set out in our guide to claiming GST.
Use case: Hamilton electrical retailer
Situation: On the invoice basis, a store returned GST on a $115 sale. Months later the customer has still not paid, and the debt is written off.
GST: $115 times 3 divided by 23 is $15, claimed back as a credit adjustment.
Action: Type 115. Show the $15 on the IR372 under bad debts written off, then in Box 13. More in our guide to GST on bad debts.
Use case: Australian store, Auckland buyer
Situation: A store registered for New Zealand GST sells a fascinator for NZ$23 and charges NZ$11.50 shipping, both including GST.
GST: NZ$34.50 times 3 divided by 23 is NZ$4.50, including NZ$1.50 in the shipping fee.
Action: Type 34.50. That checkout value is not the estimated customs value. Border charges are on the import duty calculator.
Fringe benefits and entertainment
GST on fringe benefits
The GST adjustment on fringe benefits starts from the taxable value of the benefits, not from the fringe benefit tax. Take out benefits that are exempt or zero rated for GST: low interest loans, other financial services, international travel, and contributions to employee superannuation and life insurance. Multiply what is left by 3 and divide by 23.
| Benefit, quarter ended 31 March 2021 | Taxable value | In the GST sum? |
|---|---|---|
| Motor vehicles | $2,605 | Yes |
| Low interest loans | $425 | No, exempt |
| Magazine and newspaper subscriptions | $52 | Yes |
| Subsidy on an employee's honeymoon trip | $600 | No, zero rated travel |
| Liable for GST | $2,657 | $2,657 times 3 divided by 23 = $346.57 |
The $346.57 goes in Box 7 of the quarterly FBT return IR420. Annual filers use Box 6 of IR422, and income year filers use Box 6 of IR421. When an employee pays the employer toward a benefit, that payment is a separate supply: $100 of cash goes into sales in Box 5 of the GST return, and the GST on it is $13.04. Companies and employers registered for FBT make this GST adjustment. Sole traders, partnerships, and trusts registered for FBT may also have a private use adjustment when an owner, partner, trust member, or associated person uses a business asset privately. How the two returns fit together is in our guide to GST on fringe benefits and the FBT return, and the benefit itself is priced on the FBT calculator.
Entertainment expenses
Usually only 50% of business entertainment is deductible for income tax, so a GST adjustment is made once a year on the part that is not. A $230 client lunch shows why the final step is not 3/23:
- Find the GST: $230 times 3 divided by 23 is $30.
- Take it off: $230 minus $30 leaves $200.
- Not deductible for income tax: 50% of $200 is $100.
- GST adjustment: 15% of $100 is $15.00.
Show the $15.00 on the GST adjustments calculation sheet IR372 under entertainment expenses, and transfer the total to Box 9. Before 1 April 2018 the last step used 3/23, which turned that $100 into $13.04. During the year, keep showing total business entertainment on the GST return. GST on private entertainment, such as a restaurant lunch with friends, cannot be claimed at all. Not for profit organisations and charities that are not liable for income tax do not make the adjustment. Timing and the 50% rules are in our guide to GST on entertainment expenses.
Rounding: invoices versus the GST return
This calculator rounds the GST to the nearest cent. The GST return works on period totals instead. On a paper return, sales and income in Box 5 minus zero rated supplies in Box 6 gives Box 7, and Box 7 times 3 divided by 23 is the GST in Box 8. Purchases in Box 11 go through the same fraction into Box 12. myIR does these calculations for you.
The return: Box 7 of $30.00 times 3 divided by 23 is $3.91.
Difference: one cent, from rounding each sale first.
Gaps of a few cents between the GST on separate invoices and the GST worked out on the period total come from this rounding, not from a wrong rate. The return screens are walked through in our guide to filing your GST return in myIR.
When the GST on a bill is not 3/23 of the total
A registered person charges 15% on most taxable supplies, either by adding GST to the price or by treating the price received as including GST. Land sales, zero rated supplies, exempt supplies, and special supplies are not charged at 15%. Check these before you split a total.
Reverse GST questions
No. Dividing the total by 1.15 gives the price before GST, and the total minus that price is the GST. It is the same sum as multiplying by 3 and dividing by 23, so both give $13.04 of GST on a $100 total. The 3/23 way is one step, with no answer to carry across.
The return works out GST on the period totals, for example Box 7 times 3 divided by 23. Rounding each invoice to the cent first can add or drop a cent. Three $10 sales hold $1.30 of GST each, $3.90 together, while 3/23 of $30 is $3.91.
No. The GST in a $5 price is 3/23 of $5, or 65 cents, which leaves $4.35 before GST. Taking 15% off would leave $4.25. The 15% applies to the price before GST, not to the total.
No. GST is not charged on sales by an unregistered person, and you can only charge GST if you are registered. Their price has no GST in it, so there is nothing for this calculator to take out.
No. GST cannot be charged on the rent for a residential dwelling, and a landlord cannot claim GST on dwelling expenses such as maintenance, rates, and insurance. Do not run a reverse GST sum on rent for a home.
Ask the seller to correct it. A seller must provide supply correction information when an incorrect GST amount is charged or GST is calculated at the wrong rate. The correction goes in the GST return for the period it was provided in.
Use the rate at the time of supply. At 12.5%, divide the total by 9. At 10%, divide it by 11. A $2,000 total at 12.5% held $222.22 of GST. This calculator only works at 15%.