How GST and QST are calculated on a Québec invoice
On a Québec invoice, the 5% GST and the 9.975% QST are both calculated on the same figure: the subtotal before tax. QST does not apply to a total that already includes GST. On a $1,000 subtotal, GST comes to $50, QST comes to $99.75, and the invoice totals $1,149.75. The effective combined rate is therefore 14.975%, not the 15.474% a stacked calculation would produce. The two taxes are added to the subtotal; they are not layered on top of each other. That is the central rule, and the most widespread error is ignoring it.
The rule dates from January 1, 2013. Before that, QST carried a nominal rate of 9.5% but was calculated on the price plus GST — genuinely cascading. The harmonization commitment behind the change is set out in information bulletin 2012-4 of Québec's ministère des Finances, published May 31, 2012: GST was to be removed from the QST base as of January 1, 2013 and, so that the removal would be revenue-neutral for Québec, the 9.5% QST rate was to be raised at that same moment to 9.975% — described there as the effective QST rate applicable until then. The amount paid therefore stayed the same: 9.5% of $1,050 equals $99.75, which is exactly 9.975% of $1,000. The odd-looking rate is the arithmetic residue of a cascade that no longer exists.
The most common mistake is keeping the old habit and applying QST to the GST-inclusive total. On a $1,000 subtotal, that produces $104.74 of QST instead of $99.75: $4.99 too much, or 0.499% of the subtotal. The gap is purely arithmetic — it is QST applied to the GST itself, or 9.975% of $50. It looks trivial on one invoice; across a year with $300,000 in taxable sales it approaches $1,500 overbilled. The total claimed that way corresponds to a combined rate of 15.474% rather than 14.975%.
The obligation to collect tax follows registration, and registration is triggered by a specific figure: $30,000 in taxable supplies, zero-rated supplies included, made worldwide by the business and its associates during a given calendar quarter or across the four preceding calendar quarters. Above that, registration for the GST and QST files becomes mandatory. Below it, the business is a small supplier and is not required to register or to collect the taxes, subject to the sectors where registration is mandatory from the first sale. A small supplier may also register voluntarily, collecting the taxes and gaining access to input tax credits and input tax refunds in return. The exact conditions, sector exceptions included, appear on the Revenu Québec page Registering for the GST and QST.
A correct invoice is more than two well-calculated tax lines: the information it must carry to serve as a supporting document is scaled by Revenu Québec to the value of the document. Depending on the bracket, that includes the date, the invoice number, a description of the goods or services, the tax breakdown, the supplier's registration numbers and the purchaser's name. The GST number carries the RT account suffix and the QST number the TQ suffix — RT0001 and TQ0001 for a first program account, RT0002 and TQ0002 for the next one. Above the first value bracket, a business customer cannot substantiate an input tax credit or an input tax refund without those numbers. The brackets in force are set out on the page Preparing Invoices.
A business proposal, by contrast, is written before tax, and that is a presentation convention rather than an oversight. The offer sets scope, schedule and investment; the itemized tax figures belong to the billing document, the one whose required information Revenu Québec describes. Noting that GST and QST apply on top is enough to signal that the amount shown is not the final amount. Blurring the two documents produces a proposal announcing a tax-included total that no later invoice will reproduce. That distinction is part of what a winning business proposal must contain.
Invoicing software either fixes this rule in its code, or gets it wrong every time. In OffrePro, both rates are constants on the invoice model — 0.05 and 0.09975 — and each is multiplied by the subtotal of the line items, never by an already-taxed total. A per-invoice switch, on by default, allows a non-taxable document. Registration numbers print only if they were entered during onboarding and the invoice is taxable. The sequential number 2026-001 is assigned on first send rather than at creation. The limits: the rates are frozen in the code and are not recorded per invoice; no registration-number format is validated; nothing prevents issuing a taxable invoice without those numbers; and the module produces no accounting export and connects to no filing service.
Three arithmetic checks are worth making before sending a Québec invoice, on top of the mandatory information described above. That QST equals 9.975% of the subtotal, not of the GST-inclusive total. That both registration numbers appear, failing which the invoice does not serve as a supporting document above the first value bracket. That the sum of the line items matches the figure the taxes were calculated on, fractional quantities such as hours being the most common source of drift. A tax rule that has been stable for thirteen years does not need to be recomputed by hand every month: it is exactly the kind of calculation worth fixing in a template, as described in automating business documents without losing your voice.