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To do a GST reverse calculation, divide the total by 1.05. A $525 price with 5% GST in it was $500 before tax, and $25 of that was GST. Don’t multiply $525 by 5%. That gives $26.25, and it’s wrong, because the 5% was charged on the smaller number.
That slip is the common one. Get the divide habit and every province works the same way.
How do I take GST out of a price?
Add the tax rate to 1 and divide the total by that number. With 5% GST the divisor is 1.05. For a total of $525, $525 divided by 1.05 is $500. The tax is the total minus the pre-tax price: $25.
The reverse tax calculator does this for any province, and it shows the split so you can copy the numbers into an invoice or an expense report. If you’re going forward instead, from a pre-tax price to a total, use the GST/HST calculator.
Which divisor fits my province?
The place of supply sets the rate, so look at where the sale is made. The CRA says Ontario’s HST is 13%, Nova Scotia’s is 14% from 1 April 2025, and the other participating provinces charge 15%. Provinces without HST charge 5% GST on its own.
| Rate charged | Divisor | Example total | Before tax | Tax inside |
|---|---|---|---|---|
| 5% GST (for example Alberta) | 1.05 | $525.00 | $500.00 | $25.00 |
| 13% HST (Ontario) | 1.13 | $1,130.00 | $1,000.00 | $130.00 |
| 15% HST (for example New Brunswick) | 1.15 | $1,150.00 | $1,000.00 | $150.00 |
| 12% in BC, GST plus PST | 1.12 | $112.00 | $100.00 | $12.00 |
What about cents that don’t divide evenly?
Most real totals don’t land on round numbers. Take $84.75 paid in New Brunswick at 15%. Divide by 1.15 and you get $73.70, with $11.06 of HST. But $73.70 plus $11.06 is $84.76, one cent over what you paid.
That’s rounding, and it’s normal. Each figure is rounded by itself, and sometimes they disagree by a cent. Keep the total you actually paid as the anchor, and let the tax figure absorb the penny if your books need to balance.
When should I back out the tax?
Mostly when you’ve got a receipt and need the tax part. Businesses that are registered for GST/HST do this when they claim input tax credits, and some do it on old invoices where only the total was recorded. Small suppliers, those at or below $30,000 of taxable sales over four consecutive calendar quarters, aren’t required to register, so they wouldn’t charge it in the first place.
Also, and this is where people trip, not every receipt has GST in it. Basic groceries are zero-rated, which means 0% GST/HST, in every province and territory. Dividing a grocery total by 1.05 would invent a tax that was never charged.
Mistakes to avoid in a reverse calculation
Multiplying instead of dividing is the big one.
Using 5% in an HST province is another, and so is using 12% in BC on an item that was exempt from PST, where the receipt only carried 5%. Both make the pre-tax price look wrong by a few dollars on a few hundred, which is enough to fail a reconciliation.
That last one affects our own BC setting. The GST and PST calculator applies the full 12% to whatever you enter, so for a PST-exempt item divide by 1.05 yourself. In Quebec, the QST has its own formula, handled by the GST and QST calculator.
Where the numbers come from
Rates come from the Canada Revenue Agency’s GST/HST pages, read on 30 September 2026, including the Nova Scotia change of 1 April 2025 and the zero rate on basic groceries. The $30,000 small supplier figure comes from the CRA’s registration page. The CRA page we used didn’t show the split of HST into federal and provincial parts for every province, so we don’t give one here.
Frequently asked questions
How do I calculate GST backwards?
Divide the total by 1 plus the rate. At 5% GST, divide by 1.05, then subtract the result from the total to get the tax.
Why isn't the tax 5% of the total?
Because the 5% was charged on the price before tax, which is smaller than the total.
What divisor do I use in Ontario?
1.13, since Ontario's HST is 13% according to the CRA.
Why is my answer a cent off?
Each figure is rounded on its own. Keep the amount you paid as the fixed number and adjust the tax by the cent.
Does this work for groceries?
No. Basic groceries are zero-rated, so there's no GST inside the price to remove.
- GST, HST, PST and QST explained for every province
Which GST, HST, PST or QST applies where you shop or sell in Canada, the 2026 rates for every province, who must register and how to add or remove tax
Information only. This page is general information, not tax, legal or accounting advice. Tax-Services.ca is an independent publisher: we don't prepare or file tax returns or offer tax services, and we have no connection with the CRA, Revenu Québec, any government or any company named on this page. Check the figures that matter with an official source or a qualified professional.