How Loan Interest Works: A Plain-English Guide
When you borrow money, the lender charges interest for the use of it. Understanding how that interest is calculated helps you compare offers and decide how long to borrow for.
Written by the Toolseza team · Updated 2026-10-01
The three numbers that decide the cost
Every loan comes down to the amount borrowed (the principal), the interest rate and the term. A fixed monthly payment is calculated so that, after the last payment, the balance is exactly zero.
Why early payments are mostly interest
Interest is charged on the balance you still owe. At the start the balance is at its highest, so most of the first payment pays interest and only a little reduces the principal. As the balance falls, the interest part shrinks and the principal part grows. This is called amortisation.
A worked example
Borrow 10,000 at 6% a year for three years. The monthly rate is 0.5% and there are 36 payments. The payment is about 304.22. In the first month, interest is 50 (0.5% of 10,000), so only about 254 reduces the debt. Over the full term you pay about 10,952, so the interest cost is roughly 952.
Term versus rate
A longer term lowers the monthly payment but raises the total interest, because the balance stays higher for longer. The same 10,000 at 6% over five years has a payment of about 193.33 but costs about 1,600 in interest. A lower rate always reduces cost, so shopping for the best rate matters more the longer the loan runs.
Look beyond the headline rate
Fees, insurance and early repayment charges can add to the real cost. The annual percentage rate (APR) is designed to include mandatory costs, so compare APRs between lenders. Ask whether you can make extra payments without penalty, since overpaying reduces the balance and the interest.