Simple interest and compound interest: two different arithmetics
Simple interest is charged only on the amount you originally borrowed. The arithmetic is principal multiplied by rate multiplied by time. If you never repay any principal, the interest charged in each period stays the same, because the base never changes.
Compound interest is charged on the balance including interest that has already been added to it. Each period, unpaid interest joins the principal, and the next period's interest is calculated on that larger figure. Two loans can carry the same quoted rate and still cost different amounts, because the compounding frequency and your payment behaviour change how often interest is added to the balance.
Compounding frequency is written into the agreement. A nominal rate compounded more often produces a higher effective annual cost than the same nominal rate compounded less often. When you compare two products, compare effective annual rates, because a headline rate without a compounding basis is only half the information.
The semi-annual convention for fixed-rate mortgages
Canadian fixed-rate mortgages are compounded semi-annually by law. The nominal rate on your mortgage commitment is applied twice a year, with interest added to the balance at those points, rather than monthly or daily. The practical effect is that the effective annual rate on a fixed-rate mortgage is slightly higher than the nominal rate you were quoted.
This is one reason a mortgage rate and a line of credit interest rate cannot be compared side by side without adjustment. They are quoted on different compounding bases. Converting both to an effective annual rate puts them on the same footing.
For products other than fixed-rate mortgages, the compounding and calculation method is set out in the agreement and in the disclosure document you receive before you sign. That disclosure is where you find the base for the calculation, the compounding basis, and whether interest is calculated on a daily balance or a periodic one.
Why the balance matters more than the rate
Interest for any period is the rate multiplied by the balance. Change the balance and the interest changes in proportion. Change the rate and interest also changes in proportion, but the range of rates actually available to any one borrower is narrow, while balances can differ by multiples.
Put simply: halving the balance on a loan at a given rate halves the interest for that period. The lowest rates are only available to the most qualified applicants. If the best advertised rate is not on offer to you, comparing yourself to it does not reduce what you owe; reducing the balance, or the time you carry it, does.
Time is the third variable, and the one people underestimate. Interest accrues for as long as a balance is outstanding, so the total cost of a loan depends on how long you carry it, not only on the rate attached to it. On an amortizing loan, a large share of each early payment goes to interest rather than principal, which is why an extra payment made early in the schedule reduces the total more than the same amount paid later. The rules on extra payments, including any charge for prepaying, differ by lender and are set out in your contract.
How line of credit interest is calculated
A line of credit is revolving rather than amortizing. You draw funds, repay, and draw again up to an approved limit. Interest accrues on the outstanding balance, and the balance can change from day to day. That is why line of credit interest is often calculated on a daily balance: the lender applies a daily rate to whatever you owe that day, then totals it for the statement period.
Because of that, the timing of your payments matters. A payment made earlier in the period reduces the balance for more days and therefore reduces the interest charged for that period. A payment made near the end of the period affects the same statement less. Whether your agreement uses a daily balance or an average daily balance is stated in your credit agreement.
For secured lines of credit at federally regulated lenders, a home equity line of credit is generally limited to 65% of appraised property value, with total secured lending against the property usually capped at 80%. Those are limits on how much can be secured, not a statement about what any particular borrower will be offered.
How to run the numbers yourself
An interest on loan calculator is only as accurate as the figures you feed it. Before you use one, collect the following from your agreement, your disclosure statement or your statement of account:
- The principal, meaning the amount actually advanced rather than the amount you asked for.
- The nominal rate and the compounding basis: annual, semi-annual, monthly or daily.
- The payment amount and the payment frequency.
- The amortization period and the term, which are not the same thing.
- Whether the rate is fixed or variable, and what a variable rate is tied to.
- Any fees added to the balance or charged separately.
- The prepayment rules, including any charge for paying early.
A loan interest calculator that assumes monthly compounding will produce a different figure from a fixed-rate mortgage that compounds semi-annually, even when every other input is identical. If the output surprises you, check the compounding basis first, because that is the most common mismatch.
When you use an interest on loan calculator, treat the output as an estimate of arithmetic, not a quote. It tells you what the numbers do under the assumptions you entered. It cannot tell you what a lender will offer you.
What sets the outer limits on the cost of borrowing
Canada does not leave the cost of borrowing entirely to the market. The criminal rate of interest is 35% per year under Criminal Code s. 347 on the Justice Laws Website, which sets a ceiling on what can lawfully be charged, subject to the specific rules and exemptions in that section and to how the courts have applied them.
Payday lending is handled separately. Where a province operates a licensed payday lending regime, the federal Payday Lending Regulations (SOR/2024-114) cap the cost of borrowing at $14 per $100 advanced. Some provinces set a payday cap lower than $14 per $100, and the lower cap applies. Quebec does not license payday lending, which effectively prohibits the model there. A payday loan is generally up to $1,500 for a term of 62 days or less.
Benchmark rates come from a different source. The Bank of Canada — rates publishes the policy interest rate, the prime rate, conventional mortgage rates and Government of Canada benchmark bond yields. These are benchmarks, not offers, and no lender is obliged to lend at them. A lender sets its own rate based on its cost of funds, its assessment of your file and its own lending policy.
For mortgages at federally regulated lenders, qualification rules add another layer. Federally regulated mortgage lenders generally work to a total debt service ratio ceiling of about 44%, and qualify an uninsured mortgage at the greater of the contract rate plus 2 percentage points and 5.25%, under OSFI Guideline B-20. Those thresholds affect whether a mortgage is approved and how large it can be, which in turn determines the balance that interest is calculated on.
Comparing the main ways interest is calculated
| Type of interest | How interest is calculated | What changes the total cost | Where the details are set out |
|---|---|---|---|
| Simple interest | Rate applied to the original principal only, for the time it is outstanding | Rate, principal, and how long you take to repay | Loan agreement and disclosure statement |
| Compound interest | Rate applied to the balance, including interest already added to it | Compounding frequency, rate, balance, and payment timing | Loan agreement and disclosure statement |
| Fixed-rate mortgage | Compounded semi-annually by law, with payments spread across the amortization | Balance, amortization, extra payments, and the rate at renewal | Mortgage commitment and mortgage disclosure |
| Line of credit | Accrues on the revolving balance, often calculated daily | Daily balance, payment timing, and changes in the variable rate | Credit agreement and monthly statement |
Who regulates what, and where a complaint goes
Lending in Canada is licensed provincially, so the regulator and the rules differ depending on where you live and who you borrow from. Federally regulated financial institutions' consumer complaints go to the Financial Consumer Agency of Canada; provinces license and supervise most other lenders.
If a debt has become unmanageable, the rules are separate again. Only a licensed insolvency trustee can administer a consumer proposal or bankruptcy. A consumer proposal stays on a credit report for 3 years after completion, or 6 years from filing, whichever comes first, and a first bankruptcy stays on a credit report for 6 years after discharge. Canada has two national credit reporting bureaus, Equifax Canada and TransUnion Canada, and your borrowing history with licensed lenders is reported to them.
Where loanmoose.ca fits
loanmoose.ca is not a lender and does not make credit decisions. It is a matching and comparison service: it helps you understand what kinds of borrowing exist, how interest on each is calculated, and which licensed providers may suit your situation. The rate, the term and the approval decision come from the lender.
Understanding the arithmetic is worth doing before you apply, because the structure of a product, whether simple or compound, amortizing or revolving, fixed or variable, decides how the cost moves as your balance and the benchmark rates change. For significant borrowing decisions, and for anything involving tax or insolvency, get advice from a regulated professional who can review your whole file.