How to Calculate GST and QST on Your Invoices Correctly — Montreal CPA Firm Near You

Calculating GST and QST correctly is one of the simplest yet most misunderstood parts of running a business in Quebec. A single miscalculation can trigger reassessments, customer disputes, or refund denials. QST has a unique rule: it must be calculated on the GST-inclusive amount — not simply on the base price. Most POS systems and U.S.-based software platforms miscalculate QST unless configured manually. This guide explains exactly how to calculate GST and QST properly and avoid costly errors.

Why Proper Calculation Matters
Incorrect QST calculation results in under-collected tax — and Revenue Québec will still require you to remit the correct amount. That means you pay the difference out of pocket. For businesses processing hundreds or thousands of transactions, small calculation errors accumulate fast.

Legal and Regulatory Framework
GST calculation rules fall under the Excise Tax Act; QST rules under the Tax Administration Act. Quebec requires QST to be applied on the GST-inclusive value. Canderel (1998) enforces accuracy in financial reporting. Lac d’Amiante (2001) confirms RQ can reassess when tax is miscalculated.

 What Happens When QST Is Calculated Incorrectly
Imagine running an online store with 2,000 sales a month. Your checkout platform charges QST incorrectly — on the base price instead of the GST-inclusive amount. After a year, RQ audits. They calculate that you under-collected QST by $12,000. You must now pay the difference plus interest, even though customers were only charged the wrong amount. The auditor informs you that your software settings triggered the issue — but the responsibility is yours.

Learning Section: Correct Calculation Method
Step 1: Calculate GST at 5% of the sale price.
Step 2: Add GST to the sale price.
Step 3: Calculate QST as 9.975% of that new total.
Example:
Price: $100
GST: $5
QST = 9.975% × ($100 + $5) = $10.47
Total = $115.47

SEO Learning: Primary and Secondary Keywords
Primary: calculate QST correctly Quebec, GST QST invoice calculation, Montreal CPA tax help.
Secondary: QST on GST-inclusive amount, invoice tax rules Quebec, POS tax setup Quebec.

Documentation Section: What You Must Keep
Calculation worksheets
POS configuration screenshots
Invoice templates showing tax
Accounting system tax codes
Reconciliation reports
These documents protect you during audits.

Common Mistakes
Charging QST only on base price
Incorrect POS/e-commerce settings
Not updating tax codes after software changes
Failing to show taxes separately on invoices
Incorrect rounding rules

Winning With Revenue Québec and Canada
Mackisen configures tax settings in QuickBooks, Xero, Sage, Shopify, Square, and custom POS systems to ensure proper tax calculation. We test transactions, reconcile GST/QST accounts, and audit each setup to prevent costly errors.

Mackisen Service Hub
We provide full invoicing setup, POS configuration, e-commerce tax optimization, tax testing, and audit support.

Why Mackisen
With over 35 years of tax compliance expertise, Mackisen ensures your invoices always calculate GST/QST correctly, protecting your business from reassessments and penalties.