How to Calculate Tax Backwards From a Total (Reverse GST/HST)

Receipts, expense claims and bookkeeping often give you only the total. To split it into price and tax, you work backwards.

The formula

• Price before tax = total ÷ (1 + rate)
• Tax = total − price before tax

Why you can’t just take the percentage off

Tax was calculated on the smaller, pre-tax price. Taking 13% of a $113 total gives $14.69, but the HST is actually $13.00 — because it was 13% of $100. Dividing by 1.13 undoes the original multiplication exactly.

Divide by this number

Rate	Divide the total by	Example: $100 total
5% GST	1.05	$95.24 + $4.76 tax
11% (SK)	1.11	$90.09 + $9.91 tax
12% (BC, MB)	1.12	$89.29 + $10.71 tax
13% HST (ON)	1.13	$88.50 + $11.50 tax
14% HST (NS)	1.14	$87.72 + $12.28 tax
14.975% (QC)	1.14975	$86.98 + $13.02 tax
15% HST (NB, NL, PE)	1.15	$86.96 + $13.04 tax

Splitting GST and PST

In provinces with two taxes, first find the price before tax using the combined rate, then multiply that price by each rate.

Example — Manitoba, $560 total: $560 ÷ 1.12 = $500 before tax. GST is $500 × 5% = $25 and RST is $500 × 7% = $35.

When the answer is a cent off

Because stores round tax to the nearest cent, a reverse calculation can land a cent away from the original price. The reverse tax calculator keeps your total exactly and puts any leftover cent into the tax, so the parts always add up.

Common uses

• Recording expenses and input tax credits from receipts — see the GST/HST remittance calculator.
• Checking that a “tax included” price was calculated correctly.
• Working out a tip on the pre-tax amount of a restaurant bill.