How compound interest actually works (with examples)

Compound interest is often called one of the most powerful forces in personal finance, but the idea behind it is simple: you earn interest not only on the money you put in, but also on the interest that money has already earned. Over enough time, that snowball effect can turn modest, regular savings into a surprisingly large sum.

The basic idea

Imagine you put $1,000 into an account earning 7% per year. After one year, you have $1,070 — the original $1,000 plus $70 in interest. In the second year, you don't just earn 7% on your original $1,000; you earn 7% on $1,070, giving you $1,074.90 in interest for that year alone. The gap between simple growth and compound growth widens every year, which is why long time horizons matter so much.

Why starting early beats saving more

Two people, both aiming to retire at 65, illustrate this well. One starts investing $200 a month at age 25. The other starts at age 35 but invests $300 a month to "catch up." Even though the second saver puts in more money overall, the first saver — thanks to ten extra years of compounding — typically ends up with a larger final balance. Time in the market tends to matter more than the size of each contribution.

Compounding frequency matters, a little

Interest can compound annually, quarterly, monthly, or daily. More frequent compounding produces slightly higher returns for the same stated annual rate, but the difference is usually small compared to the effect of the rate itself and the length of time invested. Don't let compounding frequency distract you from the two factors that matter most: your rate of return and your time horizon.

Try it yourself

Our compound interest calculator lets you plug in a starting amount, an interest rate, a timeframe, and even monthly contributions, so you can see exactly how your own numbers might grow.

This article is for general education and does not constitute investment advice. Actual returns vary and are never guaranteed.