If you've used a US mortgage calculator to estimate a Canadian payment, you may have noticed the number doesn't match what your bank quoted. That's not a broken calculator — it's because Canada and the US use different compounding methods.
Canada's Statutory Method: Semi-Annual Compounding
Canada's Interest Act requires residential mortgages to compound semi-annually. That means interest compounds every six months, not monthly.
So if the annual rate is 4.5%, the monthly rate is not simply 4.5% ÷ 12 = 0.375%. It must be converted using the following formula:
For 4.5%:
- Semi-annual rate = 4.5% ÷ 2 = 2.25%
- Monthly rate = (1 + 0.0225)1/6 − 1 ≈ 0.3712%
If you use US-style monthly compounding (simply dividing by 12), the monthly rate is 0.375%. The difference looks small, but over 25 years on a $600,000 mortgage, the total interest difference can reach thousands of dollars.
Canada vs US: Compounding Comparison
| Item | Canada | US |
|---|---|---|
| Statutory method | Semi-annual compounding | Monthly compounding |
| Legal basis | Interest Act | State law varies |
| Monthly rate conversion | (1 + r/2)^(1/6) − 1 | r ÷ 12 |
| Effect on borrowers | Slightly lower payment at same annual rate | Slightly higher payment |
Why This Matters
1. Accuracy of payment estimates
Using the wrong compounding method creates payment estimate errors. The monthly difference is small, but for mortgage approval and budgeting, accurate numbers matter.
2. Stress test calculation
The stress test uses the higher of "contract rate + 2%" or "5.25%". If the compounding method is wrong, the stress test result will also be inaccurate.
3. Comparing mortgage options
When comparing quotes from different lenders, ensure all calculations use the same compounding method, or you may make the wrong decision.
Does This Apply to Variable and Fixed Rates?
Yes. Both fixed-rate and variable-rate Canadian residential mortgages use semi-annual compounding. When the variable rate's benchmark (such as prime) changes, payments adjust accordingly, but the compounding method remains semi-annual.
How to Verify Quickly
You can use our mortgage calculator, which already implements the Canadian statutory semi-annual compounding method. To verify manually:
- Divide the annual rate by 2 to get the semi-annual rate
- Take (1 + semi-annual rate) to the power of 1/6, then subtract 1, to get the monthly rate
- Use the monthly rate and number of payments in the annuity formula to calculate the payment
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