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If you compare a Canadian mortgage with an American one at the same posted rate, the Canadian version costs slightly less in interest. The reason is a quirk of Canadian law: fixed-rate mortgages in Canada compound semi-annually, while US mortgages compound monthly. It's a small difference, but on a six-figure loan over decades it's real money — and it's the single most misunderstood number in Canadian mortgage math.
What "compounding frequency" actually means
Compounding frequency is how often interest is calculated and added to the balance. The more often interest compounds, the more you pay, because you start paying interest on previously accrued interest sooner. A rate of "5% compounded monthly" is therefore slightly more expensive than "5% compounded semi-annually," even though both are quoted as 5%.
Canada's Interest Act requires that mortgage interest be disclosed as an annual rate compounded no more than twice a year. In practice, lenders quote fixed mortgage rates as a nominal annual rate compounded semi-annually (twice per year). Your payments, however, are usually made monthly, biweekly, or weekly — so the lender has to convert that semi-annual rate into an equivalent periodic rate for your payment schedule.
The math behind the monthly rate
To turn a semi-annual nominal rate into the effective monthly rate used for your payments, lenders use this conversion:
monthly rate = (1 + annual rate ÷ 2)(1/6) − 1
The ÷ 2 reflects the two compounding periods per year, and the 1/6 spreads each half-year across six monthly payments. Contrast that with a US mortgage, where the monthly rate is simply annual rate ÷ 12. That small structural difference is why the same headline rate produces a slightly lower effective annual rate in Canada.
A worked example
Take a $500,000 mortgage at 5.00% amortized over 25 years.
- Canadian (semi-annual): monthly rate = (1 + 0.05 ÷ 2)1/6 − 1 ≈ 0.412%. The monthly payment works out to roughly $2,908.
- US-style (monthly): monthly rate = 0.05 ÷ 12 ≈ 0.417%. The monthly payment is roughly $2,923.
The Canadian structure saves about $15 per month here — around $4,500 over the full amortization. The effective annual rate on the Canadian mortgage is about 5.06%, versus 5.12% for monthly compounding. Not life-changing, but it's why you can't just divide a Canadian rate by 12 and expect the right payment.
Where semi-annual compounding does and doesn't apply
Semi-annual compounding is the convention for fixed-rate closed mortgages. Variable-rate mortgages and home equity lines of credit (HELOCs) are typically tied to the lender's prime rate and often compound monthly, because they're calculated on the outstanding daily balance. When you compare offers, check the compounding basis, not just the number — a variable rate quoted at the same figure as a fixed rate isn't a perfectly clean comparison.
Why this matters when you shop for a mortgage
Two practical takeaways. First, when a lender or a US-based calculator gives you a monthly payment, make sure it's using the Canadian semi-annual convention — otherwise the payment (and the total interest) will be a little too high. Second, the "effective annual rate" (EAR) is the number that lets you compare offers with different compounding fairly. A mortgage at 5.00% semi-annual and one at 4.99% monthly are almost identical in real cost, so don't over-weight tiny differences in the posted rate.
Frequently asked questions
Do all Canadian mortgages compound semi-annually? Fixed-rate closed mortgages do, by convention under the Interest Act. Variable-rate mortgages and HELOCs are usually calculated on the daily balance and compound monthly.
Does semi-annual compounding make my rate lower? Not the posted rate, but the effective annual rate is slightly lower than the same figure compounded monthly, which marginally reduces your payment and total interest.
Why do Canadian and US mortgage calculators disagree? Most US calculators divide the annual rate by 12. A Canadian mortgage needs the semi-annual conversion, so a US calculator will slightly overstate your Canadian payment.