What Is Compound Interest? How Money Grows on Its Own Growth

Compound interest is interest paid on your principal and on the interest that has already built up. Investor.gov defines it simply: interest paid on principal and on accumulated interest. That small difference from simple interest is what makes long-term saving and investing so powerful.

How does compounding actually work?

With simple interest, you earn a return only on your original deposit. With compound interest, every period's earnings get added to the base, so the next period earns on a bigger number. The growth is slow at first, then speeds up. The longer the money stays invested, the more dramatic the effect.

Why does starting early matter so much?

Time is the ingredient that does the heavy lifting. Two people can invest the same monthly amount at the same return, and the one who starts ten years earlier can end up far ahead, because their money has more compounding periods behind it. You can test this yourself with the SEC's free compound interest calculator at Investor.gov, which lets you enter a starting amount, monthly contribution, rate and years.

Where does compounding show up in real life?

Interest on savings accounts compounds. So does growth inside a brokerage account when you reinvest dividends and gains. A cash gift invested for a child at birth gets roughly 18 years of compounding before college, which is why many families think of gift money as seed money for long-term goals rather than something to spend right away.

Frequently asked questions

Is compound interest guaranteed? No. In a savings account the rate is set by the bank and can change. In investments, returns vary and investments can lose value. Compounding describes how growth builds, not a promise of returns.

Does compounding work against me too? Yes. Debt compounds the same way. Credit card balances and loans grow on top of accumulated interest, which is why carrying high-interest debt is so costly.

Sources and further reading: Investor.gov, "Compound Interest" (https://www.investor.gov/introduction-investing/investing-basics/glossary/compound-interest); Investor.gov, Compound Interest Calculator (https://www.investor.gov/financial-tools-calculators/calculators/compound-interest-calculator).