Compound Interest

Compound interest is earning returns on your prior returns, not just your original money — the engine behind long-term growth.

Why it matters

It is why starting early beats investing more later; time is the multiplier.

Worked example

Common confusion

Compounding works both ways — it also deepens losses and debt if working against you.

How investors use it

Reinvest dividends and keep fees low so compounding works on the full balance. Pay down high-interest debt first, because the compound penalty on debt almost always exceeds what you can earn investing.

A real-world scenario

Two siblings start saving at 25. One invests $5,000 a year for ten years, then stops and lets it ride. The other starts at 35 and invests $5,000 a year for 30 years straight. Assuming 8% returns, the early starter’s $50,000 of contributions grows to roughly $680,000 by 65 — more than the late starter’s $150,000 of contributions, which reach about $610,000. The ten-year head start is worth more than three times the money, purely because compounding had longer to work. Time, not amount, is the lever.

Key takeaway

Compound interest is the engine of long-term wealth, and time is its fuel. Starting early beats starting rich, because each extra year multiplies the entire growing balance, and the same math that builds wealth quietly doubles debt and deepens losses when it works against you. Reinvest returns, keep fees low, and pay down high-interest debt before investing, because the compound penalty on debt almost always beats the compound reward on savings. The rule of 72 makes the math instant.

Definitions reviewed by the Investing Glossary editorial team.