How the GST/HST Reverse Calculator Works

Why you can't just subtract the tax rate

Say a client tells you their budget is $1,130, tax included, in Ontario (13% HST). It's tempting to subtract 13% of $1,130 ($146.90) and call the pre-tax amount $983.10 — but that's wrong. Subtracting a percentage of the total removes too much, because the total already has tax baked into every dollar, not just into the pre-tax portion.

The correct formula

The pre-tax amount is the total divided by (1 + the tax rate), not the total minus a percentage of itself:

preTaxAmount = totalAmount / (1 + combinedRate)

Worked example

$113.00 total, Ontario, HST 13%:

preTaxAmount = 113.00 / 1.13 = 100.00
tax = 113.00 − 100.00 = 13.00

Scaled up, a $1,130 tax-included total works out to a $1,000 pre-tax amount — not the $983.10 the naive subtraction method above would give you. The gap between the two methods grows as the tax rate goes up.

When you'd use this

• A client gives you a fixed, tax-included budget and you need to know how much of it is actually yours before tax.
• You're checking that a receipt or invoice's tax breakdown adds up correctly.
• You're quoted a price that already includes tax and need the pre-tax figure for your own bookkeeping.

The formula is simple once you know it, but provinces that charge two separate taxes (GST + PST, or GST + QST) split the tax portion between two rates, and any rounding needs to reconcile to the cent — exactly the kind of detail a calculator handles automatically instead of by hand.