Canadian mortgages work differently from US, UK, and Australian mortgages in one important, legally mandated way: how interest compounds. Get this wrong in a calculator and every downstream number — your payment, your amortization schedule, your extra-payment savings — will be slightly off. Here's what's actually going on, and what to know before you pay extra.
Why Canadian mortgages compound semi-annually
Under Canada's federal Interest Act, any mortgage with a term of more than five years, and in practice virtually all fixed-rate mortgages, must express and calculate interest as compounded semi-annually, not in advance — even though you still make payments monthly. This is a legal requirement, not a lender preference.
To turn that semi-annual nominal rate into the effective monthly rate used to calculate your payment, lenders:
- Divide the nominal annual rate by 2 to get the semi-annual rate.
- Compound that twice to get the true effective annual rate: (1 + semi-annual rate)² − 1.
- Convert the effective annual rate into an equivalent monthly rate: (1 + effective annual rate)1/12 − 1.
The result is a monthly rate that is slightly lower than what you'd get by naively dividing the nominal annual rate by 12 (the method used in the US, UK, and Australia). On a typical loan, the difference is small per month but compounds meaningfully over decades.
Worked example
Take a $400,000 mortgage at a 5.5% nominal annual rate over a 25-year amortization:
| Method | Effective monthly rate | Monthly payment |
|---|---|---|
| Correct Canadian (semi-annual) compounding | ≈ 0.4532% | $2,441.57 |
| Naive monthly compounding (US-style, incorrect for Canada) | ≈ 0.4583% | $2,456.35 |
That's roughly a $15/month difference on this loan from the compounding method alone — which is exactly why a generic US calculator will quietly overstate your Canadian payment and understate your extra-payment savings. Our calculator applies the correct semi-annual convention automatically whenever you select Canada.
Prepayment privileges: how much extra can you pay?
Most Canadian mortgage contracts define specific prepayment privileges — the amount you're allowed to pay above your regular payment without triggering a penalty. These typically come in two forms, often combined:
- Annual lump-sum privilege — commonly 10-20% of the original principal balance, available once per calendar or anniversary year.
- Payment increase privilege — the ability to permanently increase your regular payment amount, commonly by up to 10-20%, without penalty.
These percentages vary significantly by lender and product — always check your specific mortgage commitment or contact your lender directly, since exceeding your privilege can trigger a real penalty.
How Canadian prepayment penalties are calculated
If you exceed your prepayment privilege on a fixed-rate mortgage, the penalty is typically the greater of:
- Three months' interest on the amount prepaid, or
- The Interest Rate Differential (IRD) — roughly, the difference between your original rate and the current rate the lender could charge for the remaining term, applied to the prepaid amount. IRD penalties can be substantial when rates have fallen since you signed your mortgage.
Variable-rate mortgages typically only charge the three-months'-interest penalty, without an IRD calculation, which is one reason some borrowers accept a variable rate specifically to preserve prepayment flexibility.
Modeling your own extra payments correctly
Using the correct semi-annual convention, a $400,000 mortgage at 5.5% over 25 years, with an extra $300/month applied within your prepayment privilege, is projected to save roughly $76,000 in interest and pay off about 5 years early. Try your own numbers — selecting "Canada" automatically applies the correct compounding — in our extra payment calculator.
Key takeaways
- Canadian fixed-rate mortgages compound semi-annually by law — a lower effective monthly rate than a naive annual-rate ÷ 12 calculation.
- Check your specific prepayment privilege (often 10-20% of original principal per year) before making a large lump sum payment.
- Exceeding your privilege on a fixed-rate mortgage can trigger a three-months'-interest or IRD penalty — confirm with your lender first.
- Variable-rate mortgages generally have simpler, smaller prepayment penalties.