Compound Interest Explained Simply

Compound interest is the reason a financially boring 25-year-old can finish richer than a high-earning 40-year-old who started later. Here is what it actually is, with numbers you can check yourself.

What compound interest is

Simple interest is paid only on the money you put in. Compound interest is paid on your money plus the interest it has already earned. Each year, the base that earns interest gets bigger — so growth speeds up over time.

Why time matters more than the amount

Imagine investing $200 a month at an average 7% annual return. After 10 years you'd have about $34,000. After 20 years, about $104,000. After 30 years, about $244,000. You contributed $72,000 over those 30 years — compounding did the rest. The last decade alone added more than the first two combined.

The catch

Compounding works on debt too. A credit card balance at 20% APR doubles in under four years if you only pay interest. The same force that builds wealth quietly destroys it when it works against you.

Run the numbers yourself

Try our compound interest calculator with your own starting amount, monthly contribution, and rate. Then try doubling the time and see what happens — it is the most useful financial lesson you can give yourself in five minutes.