How Compound Interest Actually Works (With Examples)

Albert Einstein supposedly called compound interest the eighth wonder of the world. Whether he actually said that or not, the math backs it up. Understanding compound interest is the single most important concept in investing.

Simple vs compound interest

Simple interest: you earn interest on your original deposit only. Compound interest: you earn interest on your deposit AND on the interest you've already earned. Interest earning interest. That's the magic.

A real example

Let's say you invest $500 per month at 7% annual return:

• After 10 years: $86,000 (you contributed $60,000 — the rest is growth)
• After 20 years: $260,000 (you contributed $120,000)
• After 30 years: $610,000 (you contributed $180,000)

Notice how the growth accelerates. In the first 10 years you gained $26,000 in growth. In the last 10 years (year 20-30), you gained $350,000. Same monthly contribution, massively different growth — because compound interest needs time to work.

Why starting early is everything

A 25-year-old who invests $500/month until 65 will have roughly $1.2 million. A 35-year-old doing the same thing until 65 will have about $570,000. Ten years of delay costs over $600,000 — not because of the missed contributions, but because of the lost compounding time.

Try it yourself

Use our compound interest calculator to plug in your own numbers. See how small changes in monthly contributions or starting age create massive differences over time.