Compound Sales Tax: How GST and PST Stack in Canada

Canada’s sales-tax system looks like a textbook case of stacked discounts run in reverse — two tax rates applied to one purchase — but the mechanics differ by province, and getting the model wrong produces a noticeably wrong total on anything but a small purchase. Understanding whether a province charges GST and PST on the same base or compounds one on top of the other is the difference between an estimate and an accurate number.

The federal layer: GST

The federal Goods and Services Tax (GST) applies across Canada at a single national rate. On its own, it works exactly like the US sales-tax model our sales tax calculator handles: total = pretax × (1 + GST rate).

Provinces that add a separate PST

In provinces that retain their own Provincial Sales Tax (PST) — British Columbia, Saskatchewan, and Manitoba, per the Government of Canada’s GST/HST overview— both taxes are typically calculated independently on the same pre-tax price, then summed:

total = pretax × (1 + GST% + PST%)

This is not the same as sequential compounding. If GST is 5% and PST is 7%, the combined effective rate is 12% applied once to the pre-tax price — not PST applied on top of a GST-inclusive subtotal.

Provinces using HST instead

Several provinces — including Ontario and the Atlantic provinces — replace the separate GST + PST lines with a single Harmonized Sales Tax (HST) rate, administered federally by the Canada Revenue Agency. HST behaves like a single-rate tax: apply the published rate once to the pre-tax price, exactly like a US state sales tax.

Worked example: two models compared

Note the two aren’t interchangeable — a province with a 5% GST and 7% PST doesn’t automatically total to the same result as a 13% HST province. Always confirm the current model and rate for the specific province before assuming one formula applies everywhere.

Practical takeaway

When you see two tax lines on a Canadian receipt, check whether they were both computed off the pre-tax subtotal (the common case in PST provinces) rather than assuming one compounds on the other. For estimating totals ahead of a purchase, treat the combined rate as GST% + PST% applied once, and confirm against the CRA’s current published rates for the province in question.