Lesson 7: Interest: How It Works

Interest is the price of using someone else's money — or the reward for letting someone else use yours. The two sides are mirror images. When you borrow money, you pay interest to the lender. When you save money in a bank, the bank pays interest to you, because it is using your money to lend to others. In both cases, interest is usually stated as a percentage of the amount involved, over a period of time — most often a year. That yearly percentage is called the interest rate, and it is one of the most important numbers in personal finance.

To see how it works, take a simple example. Suppose you lend a friend 100 units and agree on 10% interest per year — an example rate, not a real one; actual rates vary by country and change over time. After one year your friend owes you the original 100 plus 10% of 100, which is 10, making 110 in total. That is simple interest: interest charged only on the original amount, called the principal. If the loan ran for three years at simple interest, the interest would be 10 each year, so the total would be 130. Simple interest is easy to calculate, and it is how some short-term loans and savings products work.

Compound Interest: Interest on Interest

Most savings accounts and many loans use compound interest instead. With compounding, interest is calculated not only on the principal but also on the interest already earned. Using the same example — 100 units at 10% a year — after year one you have 110, just as before. But in year two, the 10% is charged on 110, not on 100, giving 11 of interest and a total of 121. In year three, 10% of 121 is 12.10, giving 133.10. The growth speeds up each year, because the amount earning interest keeps growing. Over long periods the difference is dramatic, and Lesson 13 explores it in full. The key idea for now is the contrast: simple interest grows in a straight line, while compound interest grows in a curve that bends upwards.

Why Rates Differ

Not all interest rates are the same. Rates are higher when lending is riskier — a borrower with a poor repayment record pays more than a reliable one. Rates are higher for longer loans, because the lender's money is tied up for longer and more can go wrong. And rates move with the wider economy: central banks set a key rate that influences all other rates, so when that rate rises, borrowing becomes more expensive and saving becomes more rewarding, and the opposite when it falls. The exact rates on offer at any moment vary by country and by product — always check your local rates rather than relying on a figure from a book or a lesson. What never changes is the logic: interest rewards the lender for waiting and for taking a risk, and it charges the borrower for using money now instead of later.