Discount and Sales Tax Calculator with Reverse Tax
Discount and Sales Tax Calculator
Forward: list price through one or two stacked discounts, then tax, to the till total. Reverse: a tax-included total back to the pre-tax amount, which is the receipts question every expense claim eventually asks.
Stacked discounts multiply rather than add: 20 percent then 10 percent takes 100 to 80 to 72, a 28 percent effective discount, not 30. Tax applies to the discounted price. The reverse calculation divides by one plus the rate; subtracting the rate from the total is wrong and always overshoots.
How to use this tool
Forward mode takes a list price through one or two stacked discounts and then sales tax to the till total, itemized line by line. Reverse mode takes a tax-included total back to the pre-tax amount, which is the question every receipt on an expense claim eventually asks.
Two pieces of arithmetic that look additive and are not
Stacked discounts multiply. Twenty percent off, then a further ten percent off, is not thirty percent: 100 goes to 80, then to 72, an effective 28 percent. The second discount applies to an already-reduced price, so stacking always yields less than the sum, and retailers know it when they advertise the two numbers separately.
Reverse tax divides. A 92 total at 15 percent tax was 80 before tax, found as 92 over 1.15, never as 92 minus 15 percent, which gives 78.20 and is wrong every time. The subtraction error overshoots because it takes 15 percent of the larger, tax-included number. The same division recovers the pre-tax amount from any tax-included figure, a 113 receipt at 13 percent HST being exactly 100 plus 13 of tax.
A worked check you can repeat on paper
List 100, 20 percent off, 15 percent tax: sale 80, tax 12, total 92, saved 20. Run reverse mode on that 92 at 15 percent and it returns precisely the 80, which is the round trip the tool was verified on. The stacked case: 100 through 20 then 10 percent lands on 72, and the tool reports the effective 28 percent so the two-discount advertisement can be judged for what it is.
Mistakes that catch people out
Adding stacked discounts. As above: 20 plus 10 is 28, not 30. Fix: multiply the keep-fractions, 0.8 times 0.9.
Reversing tax by subtraction. The 78.20 error. Fix: divide by 1 plus the rate; the tool does only this.
Taxing before discounting. Jurisdictions tax the amount actually paid, the discounted price. Fix: the order here is discount first, tax second, which is the near-universal retail rule.
Comparing effective discounts across different lists. Thirty percent off an inflated list can cost more than ten percent off an honest one. Fix: compare final prices; the unit price tool exists for exactly this.
How this was checked
The forward chain was verified on the 100, 20 percent, 15 percent case producing 80, 12 and 92, the stacked case producing 72 and an effective 28 percent, and the reverse division confirmed as the exact inverse on both the 92 at 15 percent and 113 at 13 percent cases.
Sales tax rates by province
Canada runs three different sales tax systems at once, so the same purchase costs a different amount in each province. Pick a province below and the calculator above loads its combined rate.
| Province or territory | Combined rate | Made up of | Tax on 100 |
|---|---|---|---|
| Alberta | 5% | GST only | 105.00 |
| British Columbia | 12% | 5 GST plus 7 PST | 112.00 |
| Manitoba | 12% | 5 GST plus 7 RST | 112.00 |
| New Brunswick | 15% | HST | 115.00 |
| Newfoundland and Labrador | 15% | HST | 115.00 |
| Nova Scotia | 14% | HST, cut from 15 on 1 April 2025 | 114.00 |
| Northwest Territories | 5% | GST only | 105.00 |
| Nunavut | 5% | GST only | 105.00 |
| Ontario | 13% | HST | 113.00 |
| Prince Edward Island | 15% | HST | 115.00 |
| Quebec | 14.975% | 5 GST plus 9.975 QST | 114.98 |
| Saskatchewan | 11% | 5 GST plus 6 PST | 111.00 |
| Yukon | 5% | GST only | 105.00 |
Rates current for 2026. Nova Scotia fell from 15 to 14 percent on 1 April 2025 and Saskatchewan PST rose to 6 percent in 2025. Rates change, so check the CRA before relying on one for filing.
Related Calculators and Reference
See also the Percent Off Calculator for the quick version aimed at a single discount, with a shareable link for every percentage and price. Also see the GST HST Calculator: GST, HST, PST and QST for every Canadian province, added or removed.
See also the Tip Calculator for adding a tip on top of a taxed total, and seeing what that choice costs.
Where the pre-discount price came from is the Markup and Margin Calculator. Comparing the discounted offers per unit is the Unit Price Comparison Calculator, and the general percent machinery behind all of it is the Percentage Calculator.