Simple vs Compound Interest
Simple and compound interest both add a percentage to a starting amount, but they treat the interest you have already earned very differently. That one difference is what separates slow, steady growth from the snowball effect that builds real wealth.
Simple interest
Simple interest is charged only on the original amount, called the principal. It never earns interest on interest, so it grows in a straight line. The formula is principal x rate x time. It is common on short-term loans and some bonds.
Compound interest
Compound interest is charged on the principal plus the interest already added. Each period builds on the last, so the balance curves upward and gets steeper over time. This is how savings, investments and most long-term debt behave.
Side-by-side example
Put 1,000 away at 5% for 10 years. With simple interest you earn 50 a year, so 500 in total, ending at 1,500. With compound interest (compounded monthly) you end at about 1,647. Over 30 years the gap explodes: simple gives 2,500, while compound gives about 4,468. Same starting amount, same rate; the only difference is whether the interest earns interest.
Why the gap grows
Early on the two look almost identical. The difference only becomes dramatic over long periods, because compounding works on an ever-larger balance. This is exactly why starting to save early matters so much, and why high-interest debt left unpaid gets out of hand.
Which one applies to you?
It depends on which side of the money you are on. As a saver or investor, you want compound interest, and as often compounded as possible, because it works in your favour. As a borrower, you want the opposite: simple interest, or at least the shortest possible time for compound interest to act. Many car loans and personal loans use simple interest on the balance, which is borrower-friendly, while credit cards compound, which is not. Reading which one a product uses tells you a lot about whether it is on your side.
Can compound interest work against you?
Yes, and this is the part people underestimate. The same snowball that grows your savings grows an unpaid debt. Leave a balance on a high-rate credit card and the interest is added to what you owe, so next month's interest is charged on a bigger balance. That is why a card balance can feel like it never shrinks when you only pay the minimum: compounding is quietly working against you. The fix is the same lever savers use in reverse, pay more than the minimum so the balance, and the interest on it, starts falling.
Work it out
Compare them yourself with the Simple Interest Calculator and the Compound Interest Calculator. To add regular deposits, use the Investment Calculator.
Frequently asked questions
Which grows faster, simple or compound interest?
Compound, over any period longer than one payment cycle, because it earns interest on interest. The gap widens dramatically over years.
When is simple interest used?
Often on short-term loans, many car and personal loans, and some bonds, where interest is charged only on the balance. Long-term savings and credit cards compound instead.
Which is better for a borrower?
Simple interest, or the shortest time possible for compound interest to act. Simple interest on the balance is borrower-friendly; compounding on unpaid debt works against you.
Can compound interest work against me?
Yes. On unpaid debt like credit cards, compounding makes the balance grow, which is why high-rate debt gets out of hand if you only pay the minimum.