Calculate Interest on a Loan
Updated 2026-09-05 · By Michael Chen, CPA
Learn how to calculate interest on a loan and understand how APR, monthly payment, loan term, fees, and total cost affect what you really owe before you borrow.
To calculate interest on a loan, multiply the principal by the interest rate and the time period, then adjust for how the loan is repaid. For most amortized personal loans and mortgages, the interest portion of each monthly payment is recalculated from the remaining loan balance. This guide explains how to compare loan offers using interest rate, APR, monthly payment, loan term, fees, and total cost. It is general educational information, not financial advice.
The simple interest formula
Simple interest is the easiest starting point because it charges interest on the original principal only. The formula is: Interest = Principal × Annual Interest Rate × Time in Years. The annual interest rate must be written as a decimal; 6% becomes 0.06. If the loan term is in months, divide the number of months by 12. Example: Borrow $10,000 at 6% for 3 years. Interest = $10,000 × 0.06 × 3 = $1,800. If no additional fees apply, the total cost is $11,800. Simple interest works this way when the full principal remains unpaid for the whole term.
Before calculating, gather these loan details
- Principal: the amount you borrow.
- Interest rate: the annual percentage charged on the remaining balance.
- Loan term: how long you have to repay the loan.
- Monthly payment: the amount due each billing period.
- Fees: origination charges, late charges, or other costs in the loan agreement.
- APR: the annualized cost of the loan, including interest and certain fees.
Use these details to estimate interest and compare loan offers clearly.
How amortization changes your interest calculation
Most installment loans, auto loans, and fixed-rate mortgages use amortization. With an amortized loan, each monthly payment includes both principal and interest. The interest part is recalculated from the current balance, not from the original principal. To calculate one month’s interest, use: Interest = Current Principal × Periodic Rate. The periodic rate equals the annual interest rate divided by 12 for a monthly payment loan. Suppose the remaining principal is $10,000 and the annual interest rate is 6%. The monthly periodic rate is 0.06 / 12 = 0.005, so the first month’s interest is $50. If the monthly payment is $304.22, the remaining $254.22 reduces principal. This process repeats for the full loan term.
The table below shows the same $10,000 hypothetical loan at 6% annual interest, amortized monthly, with no fees included.
| Loan term | Monthly payment | Total payments | Total interest |
|---|---|---|---|
| 36 months | $304.22 | $10,951.92 | $951.92 |
| 60 months | $193.33 | $11,599.80 | $1,599.80 |
A longer loan term lowers the monthly payment but increases total interest because you are borrowing the money for a longer time.
Why APR matters when comparing interest
The stated interest rate is the base cost of borrowing, but it does not include every cost. APR, or annual percentage rate, includes the interest rate plus certain fees and expresses them as a yearly percentage. Two loans can have the same interest rate but different APRs if one charges higher fees. In general, the loan with the lower APR is the less expensive choice when the principal and loan term are the same. Comparing APR helps prevent you from choosing a loan only because the monthly payment looks small.
Use total cost, not just monthly payment, to make your decision
Total cost is the real benchmark when you calculate interest on a loan. It includes every dollar you pay: all monthly payments, fees, and finance charges. For example, a $250 monthly payment for 48 months equals $12,000 in payments. Add $500 in upfront fees, and the total cost is $12,500. Subtract the borrowed principal from the total cost to see how much you are paying in interest and fees. A lender’s quote should show the APR, interest rate, monthly payment, loan term, and fees. If the calculation is unclear, ask whether the loan is simple interest or amortized and whether paying early reduces or refunds interest. This article is general guidance, not a recommendation to choose any specific loan.
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