Albert Einstein reportedly called compound interest "the eighth wonder of the world" — whether or not he actually said it, the idea holds true: compound interest is one of the most powerful forces in personal finance, and understanding it can change how you save and invest.
Simple Interest vs. Compound Interest
Simple interest is calculated only on your original amount (the principal). Compound interest is calculated on the principal plus any interest already earned. This means your money earns "interest on interest," which causes growth to accelerate over time rather than stay flat.
The Compound Interest Formula
The standard formula is: A = P(1 + r/n)^(nt) — where P is your principal, r is the annual interest rate, n is how many times interest compounds per year, and t is the number of years. The more frequently interest compounds (daily vs. monthly vs. yearly), the faster your money grows.
Why Time Matters More Than Amount
Because compounding accelerates over time, starting early matters more than starting with a large amount. Someone who invests a smaller sum for 20 years often ends up with more money than someone who invests a larger sum for only 10 years — thanks to the extra decade of compounding.
See exactly how your money grows over time with compounding.
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- Savings accounts: Interest compounds and adds to your balance over time
- Investments: Reinvested dividends and returns compound your portfolio's growth
- Loans and credit cards: Compound interest also works against you — unpaid balances can grow faster than expected
- Retirement accounts: Long time horizons make compounding especially powerful for retirement savings
Doing these calculations by hand across multiple years is time-consuming and easy to get wrong. Our free Compound Interest Calculator shows you exact growth over any time period, instantly.