How Mortgage Interest Works in Canada
Before diving into the formula, the most important thing to understand about Canadian mortgages: interest compounds semi-annually, not monthly. This is a legal requirement under the federal Interest Act for fixed-rate residential mortgages.
In the US and many other countries, mortgage interest compounds monthly. In Canada, compounding happens every 6 months — which means you pay slightly less total interest than a US borrower at the same stated rate.
The Canadian Mortgage Interest Formula
Step 1: Convert annual rate to effective monthly rate
$$r_{monthly} = \left(1 + \frac{r_{annual}}{2}\right)^{\frac{1}{6}} - 1$$
Where $r_{annual}$ is your annual interest rate as a decimal (e.g., 5% = 0.05).
Step 2: Calculate monthly interest on your balance
$$\text{Interest}{\text{month}} = \text{Balance} \times r{monthly}$$
Step 3: Calculate principal paid in that month
$$\text{Principal}{\text{month}} = \text{Monthly Payment} - \text{Interest}{\text{month}}$$
Worked Example: $500,000 Mortgage at 5.00%
Loan: $500,000 | Rate: 5.00% | Amortization: 25 years | Term: 5 years
Step 1: Find the effective monthly rate
$$r_{monthly} = \left(1 + \frac{0.05}{2}\right)^{1/6} - 1 = (1.025)^{0.1667} - 1 = 0.004124 = 0.4124%$$
Step 2: Calculate the monthly payment
Using the standard mortgage payment formula:
$$P = \frac{B \times r}{1 - (1 + r)^{-n}}$$
Where:
- $B$ = balance ($500,000)
- $r$ = monthly rate (0.004124)
- $n$ = total payments (25 × 12 = 300)
$$P = \frac{500{,}000 \times 0.004124}{1 - (1.004124)^{-300}} = \frac{2{,}062}{1 - 0.2921} = \frac{2{,}062}{0.7079} = $2{,}913$$
Step 3: Month 1 interest breakdown
- Interest: $500,000 × 0.4124% = $2,062
- Principal: $2,913 − $2,062 = $851
- Closing balance: $500,000 − $851 = $499,149
Amortization Schedule — First 12 Months
| Month | Opening Balance | Payment | Interest | Principal | Closing Balance |
|---|---|---|---|---|---|
| 1 | $500,000 | $2,913 | $2,062 | $851 | $499,149 |
| 2 | $499,149 | $2,913 | $2,059 | $854 | $498,295 |
| 3 | $498,295 | $2,913 | $2,055 | $858 | $497,437 |
| 4 | $497,437 | $2,913 | $2,052 | $861 | $496,576 |
| 5 | $496,576 | $2,913 | $2,048 | $865 | $495,711 |
| 6 | $495,711 | $2,913 | $2,044 | $869 | $494,842 |
| 7 | $494,842 | $2,913 | $2,041 | $872 | $493,970 |
| 8 | $493,970 | $2,913 | $2,037 | $876 | $493,094 |
| 9 | $493,094 | $2,913 | $2,034 | $879 | $492,215 |
| 10 | $492,215 | $2,913 | $2,030 | $883 | $491,332 |
| 11 | $491,332 | $2,913 | $2,026 | $887 | $490,445 |
| 12 | $490,445 | $2,913 | $2,023 | $890 | $489,555 |
| Year 1 totals | $34,956 | $24,511 | $10,445 |
After 12 months, you have paid $34,956 and your balance has dropped only $10,445 — because $24,511 (70% of payments) went to interest.
Interest vs. Principal Over a 25-Year Amortization
| Year | Interest Paid | Principal Paid | Balance Remaining |
|---|---|---|---|
| 1 | $24,511 | $10,445 | $489,555 |
| 5 | $23,026 | $11,930 | $443,424 |
| 10 | $20,693 | $14,263 | $374,416 |
| 15 | $17,652 | $17,304 | $289,155 |
| 20 | $13,624 | $21,332 | $181,459 |
| 25 | $7,945 | $27,011 | $0 |
| Total | $373,800 | $500,000 |
On a $500,000 mortgage at 5.00%, you pay $373,800 in interest over 25 years — 75% of the original loan amount in interest alone.
How Rate Changes Affect Total Interest
At $500,000 over 25 years:
| Rate | Monthly Payment | Total Interest | Total Paid |
|---|---|---|---|
| 3.50% | $2,472 | $241,600 | $741,600 |
| 4.00% | $2,614 | $284,200 | $784,200 |
| 4.50% | $2,762 | $328,600 | $828,600 |
| 5.00% | $2,913 | $373,800 | $873,800 |
| 5.50% | $3,069 | $420,700 | $920,700 |
| 6.00% | $3,228 | $468,400 | $968,400 |
A 1% difference in rate costs approximately $90,000–$95,000 over 25 years on a $500,000 mortgage.
How to Calculate Daily Mortgage Interest
Banks sometimes charge a daily interest rate — for example, when your mortgage closes on a date that doesn’t align with your first payment. The daily interest calculation:
$$\text{Daily interest} = \text{Balance} \times \frac{r_{annual}}{365}$$
Note: This uses simple daily interest (not semi-annual compounding) for the interest adjustment date calculation.
Example: $500,000 mortgage closes on June 3; first payment due July 1 (28 days of interest):
$$\text{Daily rate} = \frac{0.05}{365} = 0.01370%$$ $$\text{Interest due} = $500{,}000 \times 0.01370% \times 28 = $1{,}918$$
This amount is called the interest adjustment and is paid at closing or added to your first payment.
How to Reduce Total Mortgage Interest
1. Make a lump-sum prepayment
Most Canadian mortgages allow 10%–20% of the original principal per year as a lump-sum payment without penalty. A single $25,000 prepayment on a $500,000 mortgage at 5.00% in year 1 saves approximately $42,000 in interest and cuts 2+ years off the amortization.
2. Switch to accelerated bi-weekly payments
Accelerated bi-weekly = half your monthly payment every 2 weeks. You make 26 payments/year instead of 24 equivalent, effectively adding one extra monthly payment per year. On a $500,000 mortgage at 5.00%, this saves approximately $46,000 in interest and shortens the amortization by about 2 years and 11 months.
| Payment Schedule | Annual Payments | Years to Pay Off | Total Interest |
|---|---|---|---|
| Monthly | 12 × $2,913 = $34,956 | 25 years | $373,800 |
| Bi-weekly (accelerated) | 26 × $1,457 = $37,882 | ~22.1 years | ~$327,000 |
| Weekly (accelerated) | 52 × $728 = $37,856 | ~22.1 years | ~$327,000 |
3. Increase payment at renewal
When your mortgage comes up for renewal, consider:
- Increasing your regular payment (even $200/month extra saves significant interest)
- Shortening your amortization from 25 to 20 years
- Making a lump-sum payment at renewal if you have the funds
4. Negotiate your rate aggressively
A mortgage broker can access rates from 30+ lenders. On a $500,000 mortgage, even 0.20% lower rate saves approximately $18,000–$20,000 over 25 years.
Canadian vs US Mortgage Interest: Key Difference
| Feature | Canada | United States |
|---|---|---|
| Compounding frequency | Semi-annual (every 6 months) | Monthly |
| Legal requirement | Interest Act (Canada) | Varies by state |
| Effect | Slightly less total interest | Slightly more total interest |
| Stated rate | Semi-annually compounded rate | Annual percentage rate (APR) |
At the same stated rate, a Canadian mortgage costs slightly less total interest than a US mortgage because compounding happens less frequently.