The hard part about GST and QST isn't the math. Multiplying by 1.14975 is something everyone gets to eventually. What actually costs you is the invoice itself: a missing registration number, tax calculated on the wrong amount, a construction holdback billed at the wrong time. Your customer almost never notices those mistakes. Your accountant notices every single one.

Whether you're a roofer, a plumber, an electrician, a landscaper, in cleaning services or self-employed in light construction, you charge the same two taxes as everyone else in Quebec and you fall into the same traps. This guide covers the rates, how to calculate and round them properly, what has to appear on an invoice so your customer can claim their credits, when the tax actually becomes payable, and the mistakes that show up most often in service businesses.

The GST and QST rates you need to know

Two taxes apply to the vast majority of services performed in Quebec:

• GST, the federal goods and services tax, at 5%.
• QST, the Quebec sales tax, at 9.975%.
• The combined rate of 14.975%, when you calculate both taxes in a single step.

An amount before taxes multiplied by 1.14975 gives you the total with taxes included. That's the only formula worth memorizing. One detail that matters: the 14.975% is a calculation shortcut, not a rate to display. On the invoice, GST and QST are shown separately.

Another nuance: if you're not registered for the GST and QST files, you don't have to collect these taxes, with some exceptions, notably certain taxable sales of immovables. The thresholds that make registration mandatory depend on your situation and your volume of taxable sales. Check that directly with Revenu Québec, not on a contractors' forum.

How to calculate GST and QST on an invoice

The standard method comes down to four moves:

1. Add up your invoice lines to get the amount before taxes: labour, materials, travel, debris disposal.
2. Calculate the GST: amount before taxes x 5%.
3. Calculate the QST on that same pre-tax amount x 9.975%. Never on the GST-included amount.
4. Add all three together for the total due.

A concrete example. A $3,500 job before taxes. GST comes to $3,500 x 5% = $175. QST comes to $3,500 x 9.975% = $349.13 ($349.125 rounded). The total is $4,024.13. In a single step: $3,500 x 1.14975 = $4,024.13. Same result: a fast way to sanity-check your template.

Working backwards: finding the amount before taxes

You only have a total with taxes included and you want to break it down, say for a job-site expense paid on the spot. Divide by 1.14975: $4,024.13 divided by 1.14975 gives you back $3,500. Then apply 5% and 9.975% to that pre-tax amount to recover each tax separately. Avoid the shortcut of just taking 15% off the top: on $4,024.13 it gives roughly $3,420 instead of $3,500, and that gap adds up over a full year.

On rounding, the Revenu Québec rule is clear: only fractions equal to or greater than $0.005 count as $0.01. And when an invoice covers several goods, you're allowed to calculate the taxes on the total of the prices before rounding, rather than line by line. If your lines are mostly services, check your own case with Revenu Québec. That's what explains the one or two cent differences between two invoices that look identical.

Why QST is no longer calculated on top of GST

This is the question that comes up most, and it's rooted in a real memory. Before January 1, 2013, QST was calculated on the price including GST. In other words, you paid tax on tax. In 2012, the QST rate was 9.5%.

On January 1, 2013, the mechanics changed: QST has since been calculated on the selling price excluding GST, and the rate went from 9.5% to 9.975%. For the customer, the total barely moved. On $100: before, $5 of GST then $9.98 of QST calculated on $105, so $114.98. After, $5 of GST and $9.98 of QST calculated on $100, so still $114.98. The two changes cancel each other out.

Why does this still matter today? Because an old Excel template doesn't update itself. If your spreadsheet has been around a while, it may still be applying QST to the GST-included amount. The customer overpays, your invoice doesn't reconcile with your return, and the gap surfaces at the worst possible moment.

What to put on your invoice

Here's the part that surprises just about everyone: under the GST and QST systems, no particular invoice format or special rule is imposed, except on food service establishments. There is no official template you're supposed to copy.

What is regulated is something else: the information your customer needs in order to support their input tax credit (ITC) and input tax refund (ITR) claims. That list varies with the total value of the goods or services sold. Revenu Québec publishes the exact table of required information by amount bracket, and that's where to verify it rather than trusting a blog page, this one included.

There is, however, one clear obligation: when you make a taxable sale, you have to inform your customer that GST and QST apply. Two formats are accepted, either the price is shown separately from the taxes, or the taxes are included in the price. A tax-included price is therefore not a mistake in itself. The mistake is showing nothing at all, or displaying a tax amount without saying what it covers.

In practice, the simplest approach is to always issue a complete invoice, whatever the amount. You never have to figure out which bracket you fall into, and your customer never has to call you back. Put on it, every time:

• Your business name (or the name you operate under) and your contact details.
• Your GST and QST registration numbers.
• The invoice date and a unique invoice number.
• The customer's name and the address of the job site or place the service was performed.
• A description of the work, precise enough to be understood without a phone call.
• The amount before taxes, GST and QST shown separately, then the total due.
• Payment terms: due date, deposit already paid, balance owing.

Your GST and QST registration numbers

These are the two most frequently forgotten items, and the most expensive in wasted time. Once the sale reaches the bracket where Revenu Québec requires them, a registered customer can't support their ITC or ITR without your numbers. They call you, you reissue the invoice, and you lose time over a missing field.

Copy your numbers exactly as they appear on your registration confirmation from Revenu Québec, without reconstructing them from memory. One wrong digit is as good as no number at all: the invoice becomes useless to your customer.

And if you're not registered yet? You have no number to put on the invoice, so your customer can't claim anything. An invoice showing taxes with no registration number is a bad signal to a business client, and a source of questions for you. If you're approaching the point where registration becomes mandatory in your case, confirm it with Revenu Québec before adding taxes to your invoices, not after.

When the tax becomes payable, and the holdback trap

A lot of contractors assume they collect the tax the day the customer pays. That's not the rule. The tax becomes payable on the earlier of two dates: the day the consideration is paid, or the day it becomes due. And becoming due happens fast:

• The day you first issue the invoice.
• The date shown on the invoice.
• The day you would have issued the invoice had there been no undue delay.
• The day the amount is due under a written agreement.

The consequence is direct: any tax you collected or were required to collect is considered collected, even if your customer hasn't paid it yet. An invoice sent in March and paid in June still belongs to the period in which it became payable. That's exactly why an unpaid invoice hurts twice: you're waiting on your money and you already owe the tax.

A very common special case in construction: the holdback. When part of the payment is held back as a guarantee under a law or a written agreement covering construction or renovation work, you collect GST and QST on that holdback on the earlier of the following dates, the day the holdback is paid or the day the holdback becomes payable. The holdback therefore doesn't automatically follow the rest of the invoice. Treat it as a separate deadline, recorded somewhere reliable, or it ends up forgotten six months later.

That money isn't yours. Any GST and QST amount you collect is considered to be held in trust.

This is the nuance that saves cash flow, especially in year one. The 14.975% landing in your account isn't revenue, it's money you're holding for someone else. The healthiest habit is to set it aside the moment it comes in, rather than letting it blend into day-to-day spending.

The most common GST and QST invoicing mistakes

These come up constantly in service businesses:

1. Calculating QST on the GST-included amount. A pre-2013 reflex that inflates your customer's invoice.
2. Saying nothing about the taxes. The customer has to know whether the price shown includes them or not.
3. Leaving off your GST and QST registration numbers. Your customer loses their credits and you reissue the invoice.
4. Rounding at random. And on an invoice covering several goods, rounding line by line when the calculation can be done on the total of the prices before rounding.
5. Letting a rate like 9.97%, 14.97% or 14.975% appear on the document given to the customer. The first two exist only for cash registers that can't handle three decimals, and none of the three must appear on the document confirming the sale.
6. Charging taxes before being registered, therefore with no number to display.
7. Numbering invoices inconsistently, with duplicates or gaps. It complicates every review.
8. Spending the tax money along the way, then coming up short when the remittance is due.

One reminder before going further: this guide is informational and doesn't replace tax advice. For registration thresholds, reporting periods (Revenu Québec assigns yours, generally based on your annual total of taxable sales) and any special case, the authoritative source is Revenu Québec. Your accountant knows your file.

How software prevents these mistakes

The good news is that nearly all of these mistakes disappear once your quotes, your invoices and your registration numbers live in the same place. A template that already carries your numbers never forgets them. A calculation applied automatically to the pre-tax amount never falls back into the tax-on-tax trap. Rounding done by the machine will be the same on the next thousand invoices.

That's the job of the ORYX Quotes and invoices module: your rates and numbers are set up once, GST and QST are calculated and shown separately on every document, an accepted quote becomes an invoice without retyping anything, and numbering stays continuous. The AI Copilot keeps an eye on what's dragging: unpaid invoices, quotes with no answer, amounts sitting idle. ORYX doesn't replace your accountant and doesn't file your returns. It simply makes sure the numbers your accountant receives are clean the first time.