Why Canadian mortgages compound semi-annually
Under Canada's Interest Act, any mortgage with a fixed interest rate for a term of five years or more must be calculated using semi-annual compounding, not monthly compounding — even though payments are still made monthly. This means the annual rate is first converted to an equivalent monthly rate through a two-step formula, not simply divided by 12 as with a typical US mortgage.
The semi-annual monthly rate is (1 + annual rate ÷ 2)^(1/6) − 1, which is slightly lower than the naive annual rate ÷ 12 used in monthly compounding. As a result, a Canadian mortgage at the same nominal rate has a slightly lower monthly payment and lower total interest than the same numbers would produce under US-style monthly compounding — the gap widens with a larger principal or longer amortization.
How do I calculate a Canadian mortgage payment?
Convert the annual rate to a semi-annual-compounding monthly rate using (1 + rate ÷ 2)^(1/6) − 1, then apply the standard amortization payment formula. Example: a $400,000 mortgage at 5% for 25 years has a Canadian-style monthly payment of $2,326.42, about $11.94 lower than the $2,338.36 a naive monthly-compounding calculation would give.
Steps to calculate the Canadian-style monthly payment
- Convert the annual rate to a decimal (5% becomes 0.05).
- Compute the semi-annual monthly rate: i = (1 + rate ÷ 2)^(1/6) − 1.
- Convert the amortization period to total months (n = years × 12).
- Apply the standard payment formula: Payment = P × i × (1+i)^n ÷ ((1+i)^n − 1).
- Multiply the payment by n and subtract the principal to get total interest.
Canadian mortgage formulas
Monthly rate: i = (1 + Annual rate ÷ 2)^(1/6) − 1 · Payment = P × i × (1+i)^n ÷ ((1+i)^n − 1)
- P = principal (the amount borrowed)
- Annual rate = the nominal interest rate quoted by the lender
- n = total number of monthly payments (amortization years × 12)
- i = the equivalent monthly rate after semi-annual compounding, which is slightly lower than Annual rate ÷ 12
Canadian (semi-annual) vs. US-style (monthly) payment at the same nominal rate
| Principal | Rate | Years | CA monthly payment | US monthly payment | Difference |
|---|
| $400,000 | 5% | 25 | $2,326.42 | $2,338.36 | $11.94 / month |
| $300,000 | 4% | 25 | $1,578.06 | $1,583.51 | $5.45 / month |
| $500,000 | 6% | 30 | $2,974.12 | $2,997.75 | $23.64 / month |
| $250,000 | 4.5% | 20 | $1,576.01 | $1,581.62 | $5.61 / month |
Frequently asked questions
Why does the Interest Act require semi-annual compounding?
This rule dates to 19th-century consumer protection legislation designed to make it easier to compare mortgage rates by standardizing how the quoted rate translates into actual interest owed; it applies to fixed rates on terms of five years or more, though most lenders apply the same convention to shorter terms as well.
Does semi-annual compounding make my mortgage cheaper?
At the same quoted nominal rate, yes — semi-annual compounding produces a slightly lower effective monthly rate than monthly compounding would, so both the monthly payment and total interest are marginally lower than a US-style calculation of the same numbers would suggest.
Do variable-rate mortgages in Canada also compound semi-annually?
Many lenders apply semi-annual compounding to variable-rate mortgages too, though the Interest Act technically only mandates it for fixed rates; check your specific mortgage contract for the compounding frequency stated.
Why is the difference bigger for a larger loan or longer term?
The gap between the two monthly rates is small (a few hundredths of a percentage point), but it compounds over every payment period — a larger principal or more months means more payment periods over which that tiny rate difference accumulates.
This calculator computes the principal-and-interest payment only; it does not include property tax, mortgage default insurance premiums, or other costs a lender may add to your actual payment. Consult a mortgage professional or lender for a binding quote.