Resource
Calculate GST and QST on a Quebec Invoice
Learn how to calculate GST and QST on Quebec invoices with formulas, examples, rounding notes, and common mistakes to avoid.
When you are registered for Quebec sales taxes, most taxable invoices need to show GST and QST clearly. The basic calculation is simple, but mistakes happen when taxes are compounded incorrectly, rounded too early, or applied when the business is not registered.
Current basic formula
- GST = subtotal before taxes x 0.05.
- QST = subtotal before taxes x 0.09975.
- Total = subtotal + GST + QST.
For the usual two-step calculation, QST is calculated on the subtotal before GST, not on subtotal plus GST.
Examples
| Subtotal | GST 5% | QST 9.975% | Total |
|---|---|---|---|
| $500.00 | $25.00 | $49.88 | $574.88 |
| $1,000.00 | $50.00 | $99.75 | $1,149.75 |
| $2,500.00 | $125.00 | $249.38 | $2,874.38 |
Line-by-line calculation
If all lines are taxable at the same rates, calculating tax on the subtotal is usually straightforward. If some lines are exempt, zero-rated, discounted, or taxed differently, calculate taxes by line and then total the tax amounts.
This is why invoice tools often calculate taxes per line item instead of only at the bottom of the invoice.
Rounding
Round currency to cents. Avoid rounding halfway through a multi-step calculation unless your process requires it. Small rounding differences can appear if you round each line, round by tax, or round only the final total.
The important part is consistency and a clear invoice.
Common mistakes
- Calculating QST on subtotal plus GST.
- Charging GST/QST before registration.
- Forgetting to show tax registration numbers.
- Applying taxes to exempt or zero-rated items without checking.
- Editing spreadsheet formulas and not noticing the total changed.
Fast way to calculate
Use the GST/QST calculator for quick line-item math or invoice generator when you need a finished PDF. The generator is better when the client needs the full invoice, not just the tax total.
What is the combined Quebec sales tax rate?
R: The combined amount is often described as 14.975% when both GST and QST apply, but invoice documents should still show GST and QST clearly.
Do I charge taxes if I am under the small-supplier threshold?
R: Not unless you are registered or required to register. Check your status before billing.
Should I calculate tax before or after discounts?
R: Usually after the discount when the discount reduces the taxable price. Specific cases can vary.
Create an invoice now
Move from the guide to a PDF invoice that is ready to send.