Most sales tax questions start with a price before tax and ask what the total will be. But sometimes you're working the other way around: you have a receipt total, an invoice amount, or a quoted price, and you need to know how much of it was tax and how much was the actual cost of the item. That's called reversing, or backing out, the sales tax — and it comes up constantly in bookkeeping, expense claims, and budgeting.
The formula
It's tempting to just multiply the total by the tax rate to estimate the tax, but that overstates it — the tax rate applies to the subtotal, not the total, so multiplying the total gives you too big a number. The correct approach is two steps:
- Subtotal = total ÷ (1 + tax rate)
- Tax = total − subtotal
Dividing by (1 + rate) first correctly isolates the pre-tax amount, because it undoes the multiplication that created the total in the first place.
Worked example: Ontario, 13% HST
Say a total on your receipt reads $113.00, and it's from Ontario, where the combined HST rate is 13% (0.13). Here's the math:
- Subtotal = $113.00 ÷ 1.13 = $100.00
- Tax = $113.00 − $100.00 = $13.00
So that $113 total breaks down to a $100.00 subtotal and $13.00 of HST — which checks out, since 13% of $100 is exactly $13.
A second example: Alberta, 5% GST
The same formula works regardless of whether a province blends its tax into HST or charges GST on its own. Say a total from Alberta reads $105.00, where the only tax is 5% GST (0.05):
- Subtotal = $105.00 ÷ 1.05 = $100.00
- Tax = $105.00 − $100.00 = $5.00
Same two steps, different rate, same logic.
Why the rate matters
This only works if you use the correct combined rate for the relevant province. A $113 total wouldn't reverse the same way in British Columbia (12% combined GST + PST) or Quebec (14.975% combined GST + QST) — you'd get a different subtotal and tax split in each case, because the divisor changes. That's why it's worth confirming the applicable rate before doing the math, especially for provinces where GST and PST or QST are billed separately rather than blended into HST.
Do it automatically
Rather than doing the division by hand every time, our reverse sales tax calculator does exactly this: type in a total, pick a province, and it instantly shows the subtotal and tax breakdown for any of Canada's 13 provinces and territories, using current 2026 rates. It's the same underlying calculator used across this site — including the homepage calculator and each tax-type page — just with the total field emphasized first, since that's usually what you're starting from when you need to reverse a calculation.
Common use cases
This comes up most often when: an invoice or receipt only shows a lump-sum total; you're separating the tax portion of an expense for bookkeeping or reimbursement purposes; or you're comparing a tax-included quote against a tax-excluded one and need them in the same terms. In all of these cases, the same two-step formula applies, whatever province you're working in.