There is no single GST fraction that works everywhere. Australia extracts GST by dividing by 11, New Zealand uses 3/23, Singapore uses 9/109, and Canada changes by province. This guide gives you the right fraction for each country, a side-by-side worked example, and the subtraction mistake that quietly overstates your tax credits.
Rates last reviewed: October 2026. Fractions are checked against the Australian Taxation Office (ATO) guidance on calculating GST on purchases, Inland Revenue (IRD) in New Zealand, the Inland Revenue Authority of Singapore (IRAS), and the Canada Revenue Agency (CRA).
What does calculating GST backwards mean?
Calculating GST backwards means starting from a price that already includes GST and working out how much of that total is tax and how much is the price before tax. You divide the total by one plus the GST rate to get the net price, then subtract it from the total to get the GST component.
You meet this calculation constantly as a freelancer. A supplier sends a receipt with one total and the words GST included. A client pays a round, inclusive amount. Your accounting software asks for the GST portion of an expense before it will post the claim. In each case the tax is buried inside the total, and your job is to pull it back out accurately.
The general formula never changes. Net price equals inclusive total divided by (1 + rate). GST component equals inclusive total multiplied by rate divided by (100 + rate). What changes between countries is only the rate you feed into that formula, which is why each GST country has its own shortcut fraction. Use LancerBill’s GST calculator to run the extraction instantly, or keep the fractions below for spreadsheet and mental checks.
Which GST fraction should you use for your country?
Use the tax fraction for the rate charged: Australia divides the total by 11, New Zealand multiplies by 3 over 23, Singapore multiplies by 9 over 109, and Canada uses 5 over 105, 13 over 113, 14 over 114 or 15 over 115 depending on the province. The general formula is rate divided by 100 plus rate.
This master table is the heart of the guide. Every row starts from the same net price of 1,000 in local currency, so you can see exactly how much tax each system buries inside its inclusive total.
| Country and rate | Inclusive total on a net price of 1,000 | Fraction of the total that is GST | GST inside the total | Net price |
|---|---|---|---|---|
| Australia, GST 10 percent (ATO) | 1,100 | Divide total by 11 (1/11) | 100 | 1,000 |
| New Zealand, GST 15 percent (IRD) | 1,150 | Multiply total by 3/23 | 150 | 1,000 |
| Singapore, GST 9 percent (IRAS) | 1,090 | Multiply total by 9/109 | 90 | 1,000 |
| Canada, GST only 5 percent, Alberta and the territories (CRA) | 1,050 | Multiply total by 5/105 | 50 | 1,000 |
| Canada, HST 13 percent, Ontario (CRA) | 1,130 | Multiply total by 13/113 | 130 | 1,000 |
| Canada, HST 14 percent, Nova Scotia since 1 April 2025 (CRA) | 1,140 | Multiply total by 14/114 | 140 | 1,000 |
| Canada, HST 15 percent, New Brunswick, Newfoundland and Labrador, Prince Edward Island (CRA) | 1,150 | Multiply total by 15/115 | 150 | 1,000 |
Two patterns in that table are worth remembering. First, the denominator is always 100 plus the rate, because the inclusive total is the net price (100 parts) plus the tax on it. Second, the 15 percent rows simplify to the same 3/23 fraction, since 15/115 and 3/23 are the same number. That is why the New Zealand shortcut also works for a 15 percent HST invoice from Atlantic Canada.
Tip: If you forget a fraction, rebuild it in five seconds. Write the rate over 100 plus the rate: 10 becomes 10/110, which simplifies to 1/11; 9 becomes 9/109; 13 becomes 13/113. The fraction is never the rate itself applied to the total.
For the rate and registration background in each system, see our country guides for Australia GST, New Zealand GST, Singapore GST and Canada GST and HST. The same extraction logic powers the UK version in our guide to calculating VAT backwards and forwards.
How do you calculate GST backwards in Australia?
In Australia, divide a GST-inclusive total by 11 to get the GST component, because the 10 percent rate means the total is eleven equal parts: ten parts net price and one part GST. To get the net price itself, divide the total by 1.10. The Australian Taxation Office uses this exact divide-by-11 method for purchases.
Worked example, with assumptions stated. Country: Australia. Date: October 2026. A GST-registered freelance designer in Melbourne pays a supplier invoice of A$1,650 including GST for fully taxable printing, and needs the GST credit figure for her Business Activity Statement (BAS).
- Step 1, extract the GST. A$1,650 divided by 11 = A$150. That is the GST component, and the amount she can claim as a GST credit, with a valid tax invoice.
- Step 2, find the net price. A$1,650 divided by 1.10 = A$1,500. Or subtract: A$1,650 minus A$150 = A$1,500. Both routes agree.
- Step 3, check forwards. A$1,500 multiplied by 10 percent = A$150. A$1,500 plus A$150 = A$1,650, the original total. The extraction is correct.
The ATO teaches the same method on its small business guidance: divide the total price by 11, and do not divide by 10, which it flags as the common error. LancerBill’s GST calculator applies the one-eleventh rule automatically when you choose Remove GST, which is faster than a spreadsheet when you are clearing a pile of receipts before a BAS deadline.
Now run the same skill across borders. A freelancer reconciling one month of expenses might extract A$220 in Australia (220 divided by 11 = A$20 of GST, net A$200), NZ$230 in New Zealand (230 multiplied by 3, then divided by 23 = NZ$30 of GST, net NZ$200), and S$218 in Singapore (218 multiplied by 9/109 = S$18 of GST, net S$200). Three receipts, three fractions, one method. When you send your own bills, LancerBill’s invoice generator shows the GST line separately so your clients never have to do this extraction on your invoices.
Why is subtracting the GST rate from the total wrong?
Subtracting the rate takes a percentage of the inclusive total, but GST was charged on the smaller net price, so the two bases differ. On a 110 dollar Australian total, subtracting 10 percent gives 11 dollars of supposed GST, while the correct figure is 10 dollars. The error grows with every rate and every invoice.
Watch the damage at higher rates. A NZ$115 total contains NZ$15 of GST (115 multiplied by 3/23). Subtracting 15 percent of 115 gives NZ$17.25, an overstatement of NZ$2.25 on a single small receipt. Claim input tax credits on that basis all quarter and you have claimed tax you never paid, which is exactly the kind of error a GST audit finds first.
The mirror error hits your own pricing. If you quote an inclusive price and then subtract the rate to find your earnings, you understate your net income and may raise prices you did not need to raise. Run every extraction through the fraction, or through the GST calculator, and treat any subtract-the-rate result as wrong by construction.
Watch out: Nova Scotia is the stale-table trap in Canada. Its HST rate fell from 15 percent to 14 percent on 1 April 2025, per the CRA, so older blog tables that still extract Nova Scotia tax at 15/115 overstate the tax. The current fraction is 14/114. Check the rate date before you trust any province table.
When do you need to calculate GST backwards?
You need it whenever a receipt, bill or payment shows one inclusive total with no tax split: expense claims and input tax credits, quarterly GST returns and BAS preparation, checking a supplier invoice, pricing a job from a competitor’s inclusive quote, or absorbing GST into a displayed price in Singapore.
Three freelancer situations come up most. First, expense claims: tax authorities only let you claim the GST you actually paid, and in Australia you generally need a valid tax invoice for purchases over A$82.50 including GST before claiming the credit. The divide-by-11 figure is what goes on the return, not a guess.
Second, absorbing GST. IRAS gives the worked example directly: a Singapore business that absorbs GST on a S$10,000 sale treats that sum as inclusive, accounts for output tax of S$825.69 (10,000 multiplied by 9/109), and declares a supply value of S$9,174.31. If you advertise tax-inclusive prices to win work, that fraction decides what you actually keep. Inland Revenue states the same principle for New Zealand, where the GST to be returned on an inclusive price is the consideration multiplied by 3 divided by 23, per IRD guidance on GST-inclusive pricing.
Third, checking other people’s maths. A supplier invoice that shows a GST figure you cannot reproduce with the fraction is an invoice to query before you pay it, not after your return is filed. Canada publishes its extraction fractions openly, for example 5/105 in GST-only provinces and 13/113 in Ontario, on the CRA GST/HST rates pages, and IRAS publishes the 9/109 tax fraction in its guidance on absorbing GST.
What can break a backwards calculation?
Three things break it most often: a mixed invoice where only some lines carry GST, a second tax sitting beside GST, such as a Canadian provincial tax that is not part of the HST fraction, and rounding, where extracting tax line by line and on the total gives answers a cent or two apart.
Mixed supplies come first. An Australian invoice can combine taxable lines with GST-free lines such as basic food or some health services. Extracting one-eleventh of the whole total invents GST on the GST-free part. Extract line by line instead, and skip zero-rated and exempt lines entirely: a zero-rated supply is taxable at 0 percent, so there is no GST inside its price to extract.
Separate provincial taxes come second. In British Columbia you pay 5 percent GST plus 7 percent PST, in Saskatchewan 5 percent GST plus 6 percent PST, in Manitoba 5 percent GST plus 7 percent RST, and in Quebec 5 percent GST plus 9.975 percent QST. Those provincial taxes are not HST, they follow the place of supply, and they are not extracted with a single combined fraction. Only the GST or HST portion belongs in the fractions in the master table. If you also work with UK or EU clients, the same discipline applies in reverse with LancerBill’s VAT calculator, where each VAT rate has its own fraction.
Rounding comes third. Extract tax per line and the cents can differ by a cent or two from extracting on the invoice total. Tax authorities generally accept a consistent method, so pick one, use it everywhere, and let the calculator keep your rounding consistent across a whole return period.
Rule of thumb: the fraction is always the rate over 100 plus the rate. If your shortcut does not match that shape, it is not an extraction formula, it is a guess.
Key takeaways
- There is no universal GST fraction: Australia divides by 11, New Zealand uses 3/23, Singapore uses 9/109, and Canada ranges from 5/105 to 15/115 by province.
- Every fraction is the rate divided by 100 plus the rate, applied to the inclusive total. The net price is the total divided by 1 plus the rate.
- Never subtract the GST rate from an inclusive total. It overstates the tax, inflates input tax credits, and understates your real earnings.
- On the same net price of 1,000, the buried tax is 100 in Australia, 150 in New Zealand, 90 in Singapore and 50 to 150 in Canada.
- Extract line by line on mixed invoices, keep provincial taxes in Canada separate from GST and HST, and use one rounding method consistently.
Frequently asked questions
How do I calculate GST backwards in Australia?
Divide the GST-inclusive total by 11 to get the GST component, and divide by 1.10 to get the net price. An A$1,650 total contains A$150 of GST and A$1,500 of net price. The ATO uses this divide-by-11 method, and dividing by 10 instead is the error it warns against.
How do I find the GST inside a New Zealand price?
Multiply the GST-inclusive total by 3 and divide by 23, the IRD tax fraction for the 15 percent rate. A NZ$230 price contains NZ$30 of GST, leaving a net price of NZ$200. To get the net price directly, divide the total by 1.15.
What fraction extracts GST in Singapore?
Multiply the inclusive total by 9/109, the tax fraction IRAS specifies for the 9 percent rate in force since 1 January 2024. A S$1,090 price contains S$90 of GST. If you absorb GST on a S$10,000 sale, IRAS works the output tax as S$825.69, which is 10,000 multiplied by 9/109.
Why does the Canadian fraction change by province?
Because the combined GST or HST rate changes: 5 percent GST only in Alberta and the territories (5/105), 13 percent HST in Ontario (13/113), 14 percent HST in Nova Scotia since 1 April 2025 (14/114), and 15 percent HST in New Brunswick, Newfoundland and Labrador and Prince Edward Island (15/115). The rate follows the place of supply, so use the fraction for the province where the supply is made. Separate PST, RST and QST are not part of these fractions.
Can I just subtract 10 percent to remove GST?
No. Subtracting 10 percent of an inclusive Australian total of A$110 gives A$11, but the real GST component is A$10, because GST was charged on the A$100 net price, not on the A$110 total. Always use the fraction for your country’s rate instead of subtracting the rate.
Does the backwards calculation work on GST-free or zero-rated items?
There is nothing to extract. GST-free and zero-rated supplies carry GST at 0 percent, and exempt or input-taxed supplies sit outside the GST credit system, so their prices contain no GST component. On a mixed invoice, apply the fraction only to the lines that actually carried GST.
Sources: Australian Taxation Office, Inland Revenue (New Zealand), Inland Revenue Authority of Singapore, and Canada Revenue Agency guidance cited inline. Rates last reviewed: October 2026. This guide is for general information only and is not financial, tax or legal advice.