How Loan Interest Is Calculated: A Plain-Language Explanation

Loan interest is the cost of borrowing, calculated from the principal, the rate, and the time you hold the money. Here is how those pieces fit together.

By Promissory.ca Editorial Team Reviewed against official Canadian sources Last updated: 2026-09-16

The three ingredients

Every interest calculation starts with three ingredients. The principal is the amount you borrow. The rate is the price of borrowing, expressed as a percentage over a period, usually a year. Time is how long you hold the money. Interest is what you pay for the use of the principal over that time.

In its simplest form, the relationship is interest equals principal multiplied by rate multiplied by time. If you borrowed a sum for exactly one year at a stated annual rate, the interest would be the principal times the rate. Real loans are more complex because you repay along the way, so the balance shrinks and the interest is calculated on what remains.

Simple interest vs compound interest

Simple interest is charged only on the principal. If you never repaid any of it, the interest would grow in a straight line. Compound interest is charged on the principal and on interest that has already accumulated, so the cost grows faster because you are paying interest on interest.

Most consumer loans in Canada calculate interest on the outstanding balance, which is closer to simple interest applied repeatedly as the balance changes. Credit cards and some other products can compound, which is one reason a revolving balance can be so expensive to carry. When you read a loan agreement, look at how often interest is calculated and whether unpaid interest is added to the balance.

How amortization spreads your payments

An instalment loan is amortized, which means each payment is split between interest and principal. Early in the term, the balance is large, so the interest portion of each payment is large and only a small amount goes to reducing what you owe. Later, as the balance falls, the interest portion shrinks and more of each payment goes to principal.

The payment itself stays level on many loans, but what it is made of changes over time. That is why the balance seems to fall slowly at first and then accelerates near the end of the term.

What your payment is made of over time

How daily interest and payment frequency work

Many lenders calculate interest daily on the outstanding balance, then add it up over the payment period. This matters because it means the cost accrues day by day, and the balance you carry on any given day affects what you owe. Paying earlier within a period, or paying more than required, reduces the balance sooner and therefore reduces the interest.

Payment frequency also plays a role. Paying weekly or biweekly instead of monthly means more payments per year, which reduces the balance faster and can lower the total interest. The difference is not magic; it is simply that the money reaches the lender sooner. A calculator can show how the frequency changes the total cost for your situation.

What early repayment does

Because interest is charged on the outstanding balance, reducing that balance early saves interest. When you make an extra payment, every dollar goes straight against the principal, which lowers the balance that future interest is calculated on. The effect compounds over the remaining term.

There is a catch. Some fixed-rate loans charge a prepayment penalty, and the penalty can reduce or even cancel the benefit of paying early. Before making a lump-sum payment, check the prepayment clause in your agreement. If the loan allows penalty-free early repayment, paying extra is one of the most effective ways to cut the total cost.

How payday loan costs are calculated

Payday loans work differently from instalment loans. Instead of an annual interest rate applied to a declining balance, they charge a flat fee per amount borrowed for a short period. Where a provincial regime exists, the cost is capped at $14 per $100 borrowed, the dishonoured-payment fee is capped at $20, and the maximum payday loan is $1,500.

Because the fee is charged over a very short term, the equivalent annual rate is extremely high. The federal consumer agency illustrates this with a scenario in which $500 borrowed for 14 days costs $70, which works out to roughly 365% on an annual basis. That figure is not a rate the lender quotes; it is a way of showing how expensive a short-term flat fee becomes when expressed over a year.

What APR adds to the picture

The interest rate tells you the price of the principal, but it does not include all the costs of borrowing. The annual percentage rate, or APR, folds in the interest rate plus certain fees and expresses the total as a yearly percentage. That makes it a more honest comparison tool when you are weighing two offers with different fee structures.

In Canada, the cost of borrowing also has a legal ceiling. The Criminal Code sets the criminal rate of interest at 35% APR, reduced from 48% on 1 January 2025. A loan that exceeds that limit crosses a legal line. Within the ceiling, comparing the APR of competing offers is the clearest way to see which loan actually costs less.

The takeaway

Interest is not a fixed penalty attached to a loan; it is a running cost that responds to how much you owe and for how long. Borrow less, repay faster, and check the prepayment terms, and you keep more of your money. Understanding the mechanics turns the monthly payment from a mystery into a number you can control.