How to Calculate Mortgage Interest: Formula, Examples, and Calculator

Learn how to calculate mortgage interest in Canada with the official formula, a step-by-step numerical example, and the WiseRock mortgage calculator to automate the whole process.

Published on May 19, 2026 - 12 min read

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Table of contents

1. Why Knowing How to Calculate This Changes Your Decisions
2. The Canadian Convention: A Detail That Changes Everything
3. The Official Formula in Three Steps
4. Step-by-Step Numerical Example: $350,000 at 5.25% Over 25 Years
5. The WiseRock Mortgage Calculator to Automate the Calculation
6. Three Scenarios to Compare to Save Money
7. Common Mistakes When Calculating Interest
8. Going Further: From Interest Calculation to Buying Decision
9. Conclusion: A Simple Number That Guides a Major Decision
10. Key Takeaways
11. About WiseRock
12. FAQ

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The interest calculation on a mortgage is one of the most profitable skills a buyer or investor can develop. Whether you are buying your first property, comparing two mortgage offers, or evaluating the profitability of a rental property in Quebec, knowing how to turn an advertised rate into concrete numbers - monthly payment, total interest, principal repaid - is what separates an intuitive financial decision from an informed one.

According to the Bank of Canada, the policy rate moved within a narrow range in 2025-2026, and most 5-year fixed mortgage rates sit between 4.5% and 5.5%. A 0.5-point difference can represent $30,000 or more in interest over the life of a $350,000 mortgage.

This article gives you three things: the exact formula used by your lender, a step-by-step numerical example, and a mortgage calculator that automates the whole calculation, including the full amortization schedule.

Why Knowing How to Calculate This Changes Your Decisions

The interest rate is not a technical detail. It is the real cost of your borrowing over 20, 25, or 30 years. When you know how to calculate it yourself, three things become possible.

• Compare two mortgage offers without getting lost in sales promises. A broker may sell a "good rate" that actually costs more with a different payment frequency or amortization.
• Evaluate a rental investment project. For an income property, the exact amount of interest determines net cash flow and profitability, two variables we cover in detail in our guide How to calculate rental yield.
• Understand your amortization schedule. At the beginning of a loan, most of your payment goes to interest; only over the years does principal repayment take over. Knowing where you are on that curve changes your trade-offs (prepayment, refinancing, sale).

The goal is not to replace your lender. It is to be able to have an equal-to-equal conversation with them.

The Canadian Convention: A Detail That Changes Everything

Before the formula, there is one technical point that is rarely explained clearly: in Canada, fixed-rate mortgages are compounded semi-annually, not monthly.

This is a requirement of the Interest Act (section 6) for loans secured by a mortgage. In the United States, mortgages are compounded monthly; that is one reason American calculators give slightly different results when people try to apply them here.

In practical terms, if your lender advertises 5.25%, that rate is compounded twice per year. To get the periodic rate actually applied to each monthly payment, you have to convert it. This conversion is what makes Canadian calculations a little more subtle - and it is also why a mortgage interest calculation done by hand with a simple division by 12 always gives the wrong result.

Key point: nominal rate does not equal periodic rate. A 5.25% rate compounded semi-annually is equivalent to an effective monthly rate of about 0.4328%, not 5.25% ÷ 12 = 0.4375%. It looks like a small difference; over 25 years, it changes the numbers.

The Official Formula in Three Steps

The mortgage loan interest calculation in Canada breaks down into three connected formulas.

Step 1 - Convert the Annual Rate Into an Effective Periodic Rate

This is the step where the semi-annual convention is integrated.

Formula:

i = (1 + r/2)^(2/f) − 1

• r = advertised nominal annual rate (for example, 0.0525 for 5.25%)
• f = payment frequency per year (12 for monthly, 26 for biweekly)
• i = effective periodic rate applied to each payment

For a 5.25% rate and a monthly payment: i = (1 + 0.0525/2)^(2/12) − 1 ≈ 0.004328, or 0.4328% per month.

Step 2 - Calculate the Periodic Payment

Once i is known, the periodic payment formula is universal (it is the standard annuity formula).

Formula:

M = P × i / (1 − (1 + i)^(−n))

• P = amount borrowed (initial principal)
• i = effective periodic rate (step 1)
• n = total number of payments (amortization period in years × f)
• M = periodic payment (interest + principal)

For $350,000 at 0.4328% over 300 months (25 × 12): M ≈ $2,085.71 per month.

Step 3 - Break Each Payment Into Interest + Principal

At each payment, the interest portion depends on the remaining balance.

Formulas:

Interest in payment n = Remaining balance × i
Principal in payment n = M − Interest in payment n
New balance            = Old balance − Principal in payment n

This loop is what produces the amortization schedule: payment after payment, the balance goes down, so the interest portion goes down, and the principal portion rises mechanically. At the beginning of the loan, nearly 73% of your payment goes to interest; at the end, the opposite is true.

The full mortgage interest calculation fits into these three formulas. Everything else - scenario comparisons, prepayment simulations - is only an application of these three equations.

Step-by-Step Numerical Example: $350,000 at 5.25% Over 25 Years

Take a realistic case for 2026: a buyer financing a plex in Montreal for $437,500, putting 20% down ($87,500), and therefore borrowing $350,000 at a 5.25% 5-year fixed rate, amortized over 25 years, with monthly payments.

Step 1 - Effective periodic rate

i = (1 + 0.0525/2)^(2/12) − 1
i = (1.02625)^(0.16667) − 1
i ≈ 0.00432790  (or 0.4328% per month)

Step 2 - Number of payments and monthly payment

n = 25 × 12 = 300 payments
M = 350,000 × 0.00432790 / (1 − (1.00432790)^(−300))
M ≈ $2,085.71 per month

Step 3 - First payments broken down

Payment	Monthly payment	Interest	Principal	Remaining balance
1	$2,085.71	$1,514.77	$570.95	$349,429.05
2	$2,085.71	$1,512.29	$573.42	$348,855.63
3	$2,085.71	$1,509.81	$575.90	$348,279.73
12	$2,085.71	$1,487.21	$598.50	$343,047.28
60	$2,085.71	$1,373.75	$711.97	$316,599.82
300	$2,085.71	$8.99	$2,076.73	$0

Total cost of the loan

Total paid over 25 years = $2,085.71 × 300 = $625,714
Total interest           = $625,714 − $350,000 = $275,714

Over 25 years, interest represents 44% of the total cost of your borrowing. In other words, for every $1 borrowed, you repay $1.79. That is the numerical reality behind any Canadian residential mortgage - and it is exactly the kind of information you need to have in mind before signing.

Check: if you reproduce this mortgage interest calculation with a scientific calculator, you will get the same figures to the nearest cent. If you use an American calculator compounded monthly, you will get a payment of about $2,095.75 - roughly a $3,000 error over the life of the loan. The difference comes only from the compounding convention.

The WiseRock Mortgage Calculator to Automate the Calculation

Redoing these calculations by hand for every scenario is a waste of time. The WiseRock mortgage calculator applies exactly the method described above, with the Canadian semi-annual compounding convention already built in.

What the calculator does for you:

• Automatically converts your nominal rate into an effective periodic rate.
• Calculates your payment (monthly or biweekly) to the nearest cent.
• Produces the full amortization schedule, payment by payment.
• Shows a chart of the principal vs. interest evolution over time.
• Compares three different intentions: new mortgage, refinancing, renewal.

How to use it effectively:

1. Start with your realistic scenario (the rate you actually have, the amortization you are considering).
2. Note the payment and total interest as your reference point.
3. Rerun the simulation by changing only one variable at a time (rate, amortization, or frequency) to see the isolated impact.
4. Compare total interest between scenarios - that is the figure that weighs most over the life of the loan.

This "one variable at a time" approach is the same one used in professional financial models. It avoids the classic mistake of changing three parameters at once and no longer knowing which one produced the difference.

Three Scenarios to Compare to Save Money

The real power of the calculation appears when you start comparing scenarios. Here are three variations from the same base case ($350,000 borrowed), with the results you will get using the mortgage calculator.

Scenario	Annual rate	Amortization	Monthly payment	Total interest	Difference vs. base
Base (average rate, 25 years)	5.25%	25 years	$2,085.71	$275,714	-
Better negotiated rate	4.75%	25 years	$1,989.80	$246,939	-$28,775
Amortization extended to 30 years	5.25%	30 years	$1,928.77	$344,357	+$68,643
Higher rate	5.75%	25 years	$2,184.07	$305,220	+$29,506

Three concrete lessons come out of this comparison.

• Half a rate point is worth about $29,000 over the life of a $350,000 loan over 25 years. Negotiating your rate or comparing several lenders is not a waste of time: it is probably the highest-paid financial decision of the decade.
• Extending amortization from 25 to 30 years reduces the payment by $157/month, but costs $68,643 more in interest. This is a cash-flow vs. total-cost trade-off you should make knowingly, not by default.
• The mortgage interest calculation is not linear. Doubling the rate does not double the interest, and shortening the amortization by 5 years saves much more than 20% of the interest. That is exactly why you need a calculator, not a rule of thumb.

To go further on amortization vs. payment vs. cash flow trade-offs, our mortgage calculator lets you compare a new mortgage, a refinance, and a renewal with the same method.

Common Mistakes When Calculating Interest

Four mistakes come up again and again when buyers or investors try to reproduce this calculation.

1. Dividing the Annual Rate by 12 to Get the Monthly Rate

This is the most common mistake, and it is the one made by most American calculators imported into Canada. As explained above, the Canadian convention requires semi-annual compounding. The correct formula is (1 + r/2)^(2/f) − 1, not r/f.

2. Confusing the Advertised Rate With the Effective Annual Rate (APR)

Some lenders or comparison sites show an effective annual rate (APR) that already includes ancillary fees. Others show a pure nominal rate. If you compare two offers, make sure you are comparing rates of the same nature, otherwise your numbers mean nothing.

3. Forgetting That the Balance Changes at Every Payment

Many people calculate interest by applying the rate to the initial amount for all payments. That is wrong: interest applies to the remaining balance, which goes down at every payment. That is why early payments contain a lot of interest and little principal, and the opposite happens at the end of the loan.

4. Ignoring Payment Frequency

Moving from monthly payments to "accelerated biweekly" payments is not simply paying 26 times $1,042 instead of 12 times $2,085. The capitalization calendar differs, and the real repayment period shortens by several years. To compare correctly, you have to recalculate the full amortization schedule - which the calculator lets you do in seconds.

Going Further: From Interest Calculation to Buying Decision

The interest calculation is only one piece of the puzzle. For a buying decision to hold up, you need to combine it with other analyses.

• For a principal residence in Quebec, add transfer duties to your total cost with our welcome tax calculator, and the CMHC premium if your down payment is under 20% with the CMHC premium calculator.
• For a rental property, deductible interest changes after-tax profitability. The rental investment simulator integrates that dimension, along with vacancy, property taxes, and maintenance.
• For a refinance or renewal, the remaining interest gap is what justifies (or does not justify) a contract-breaking penalty. The mortgage calculator calculates that gap with the same method used in this article.

For an overview of the Quebec market, financing conditions, and the 2026 investment thesis, also see our complete guide Investing in Rental Real Estate in Quebec in 2026.

Conclusion: A Simple Number That Guides a Major Decision

On a $350,000 mortgage, each half-point of rate is worth nearly $30,000 over 25 years. That is the definition of a number worth calculating yourself, verifying, and using as a decision filter instead of relying on intuition or a sales promise.

The mortgage interest calculation fits into three formulas. It is not reserved for financial advisors, and it has no secrets once the Canadian convention (semi-annual compounding) is properly integrated. The WiseRock mortgage calculator turns these three formulas into an operational tool: enter your numbers, read the results, compare your scenarios.

That move from concept to concrete decision is what makes the difference between a buyer who endures their financing and a buyer who steers it.

Key Takeaways

1. In Canada, fixed-rate mortgages are compounded semi-annually, not monthly. The conversion i = (1 + r/2)^(2/f) − 1 is the step most often forgotten.
2. The periodic payment is calculated with the standard annuity formula: M = P × i / (1 − (1+i)^(−n)).
3. At each payment, interest = remaining balance × i and principal = payment − interest. The balance goes down, so the interest portion does too.
4. On a typical 25-year loan, about 44% of what you repay is interest. Half a rate point represents about $30,000 over the life of the loan.
5. Extending the amortization reduces the payment but massively increases total interest. It is a trade-off to make with numbers in hand.
6. A good online calculator remains the most efficient tool for comparing several scenarios in a few minutes without risking an error.

About WiseRock

WiseRock is a Canadian platform for real estate buyers and investors. We provide calculators, market benchmarks, and clear guides to help evaluate financing, compare scenarios, and move from calculation to decision with confidence. All our tools are designed for Canadian conventions (semi-annual compounding, CMHC rules, welcome tax in Quebec).

FAQ

How do you calculate mortgage interest in Canada?

You first convert the nominal annual rate into an effective periodic rate with the formula i = (1 + r/2)^(2/f) − 1 (where f is the payment frequency per year), then apply the annuity formula M = P × i / (1 − (1+i)^(−n)) to get the payment. At each payment, the interest portion is the remaining balance multiplied by i.

Why can't you simply divide the annual rate by 12?

Because the Interest Act (section 6) requires semi-annual compounding for loans secured by a mortgage. Dividing the annual rate by 12 would give a slightly too-high monthly rate and distort the entire amortization schedule. The difference looks tiny on one payment, but it adds up to thousands of dollars over 25 years.

What is the difference between a simple interest calculation and a mortgage interest calculation?

Simple interest always applies to the initial principal. Compound interest on a mortgage applies to the remaining balance, which goes down at every payment. That is why an amortization schedule is necessary: interest must be recalculated payment by payment, not just once.

Does the calculator work for variable-rate loans?

The calculator applies a fixed rate to the whole amortization period. For a variable-rate loan, you can simulate successive "segments" by rerunning the calculation with a new rate and the remaining balance at each change. That is the method analysts use to model a variable-rate scenario cautiously.

How can you reduce the total interest on a mortgage?

Three main levers: negotiate a better rate (half a point less is worth about $30,000 over 25 years), shorten the amortization (moving from 30 to 25 years saves tens of thousands of dollars), or make extra principal payments when the contract allows it. The mortgage calculator lets you quantify each option before deciding.

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Recommended tool - Move from reading to simulation

Run your simulation in the WiseRock mortgage calculator, then estimate the overall profitability of a rental project with the rental investment simulator.