You have a receipt for $113.00 and you need to know how much of it was tax. To reverse calculate sales tax, divide the total by one plus the tax rate. In Ontario that means dividing by 1.13. $113.00 ÷ 1.13 = $100.00 before tax, so the HST was $13.00.
This post covers the formula, the most common error, and worked examples for Ontario, British Columbia and Quebec.
Why you can’t take 13% of the total
Taking 13% of $113.00 gives $14.69. That is too high. The tax was charged on the pre-tax price, not on the total, so a percentage of the total will always overstate it.
What you want is the amount that, once tax is added, comes to exactly what you paid. That means dividing, not multiplying.
Divide by one plus the rate. Do not take a percentage of a total.
The reverse sales tax formula
This works for any single combined rate. That covers the HST provinces and any province where the provincial tax is charged on the same pre-tax price as the GST.
- Pre-tax amount = total ÷ (1 + rate)
- Tax = total − pre-tax amount
In Ontario the rate is 13%, so you divide by 1.13. Round to the cent at the end. Rounding partway through can leave your breakdown a cent off.
Reverse calculating GST and PST together
In provinces that charge a separate provincial tax, divide by one plus the combined rate. Then work out each tax on the pre-tax amount. In British Columbia that is 5% GST and 7% PST, or 12% combined.
Reverse calculating GST and QST in Quebec
Quebec works the same way, with a longer decimal. The rates are 5% GST and 9.975% QST, or 14.975% combined.
The QST is charged on the pre-tax price, not on the GST-inclusive amount. That has been the rule since 1 January 2013. A calculator that adds the QST on top of the GST is using the old method.
What to divide by in each province
| Province or territory | Combined rate | Divide the total by |
|---|---|---|
| Alberta · NT · NU · YT | 5% | 1.05 |
| Saskatchewan | 11% | 1.11 |
| British Columbia · Manitoba | 12% | 1.12 |
| Ontario | 13% | 1.13 |
| Nova Scotia | 14% | 1.14 |
| Quebec | 14.975% | 1.14975 |
| NB · NL · PE | 15% | 1.15 |
Nova Scotia has been 14% since 1 April 2025. Receipts from before that date will show 15%. For the federal and provincial split in each province, see the 2026 Canadian sales tax rate card.
Frequently asked questions
Why doesn’t my answer match the receipt exactly? Stores round tax on each item or on the whole invoice before you see the total. That rounding can put a reverse calculation a cent away from the tax printed on the receipt. The receipt’s figure is the one that was charged.
What if some items on the receipt weren’t taxed? Take those items out first. Divide only the taxable part of the total. Dividing the whole receipt will understate the pre-tax amount and overstate the tax.
How do I find the GST in a total that only has GST? In Alberta and the territories, divide by 1.05. On $105.00, the pre-tax amount is $100.00 and the GST is $5.00.
Rates change, and some goods and services are taxed differently or not at all. Verify anything you are filing against canada.ca and your provincial revenue agency. This is not tax advice.
Reverse calculating without the arithmetic
Reverse mode in Tax Moose does this for you. Enter the total and pick the province. It shows the pre-tax amount, each tax line at its own rate, and the total again as a check.
The reverse sales tax calculator is free on this site and in the app, which works offline in English and French.