Compound interest explained with real-world examples

Key takeaways

• Simple interest is paid only on your original deposit; compound interest is paid on the deposit plus interest already earned.
• Time is the biggest driver: starting early matters more than starting big.
• The same math works against you on debt, which is why high-interest balances grow so quickly.

Simple vs. compound interest

With simple interest, a $10,000 deposit at 5% earns $500 every year. With compound interest, each year’s interest is added to the balance, so the next year’s 5% is calculated on a bigger number.

The formula

Future value = P × (1 + r ÷ n)^(n × t), where P is the starting amount, r is the annual rate, n is the number of compounding periods per year and t is the number of years. More frequent compounding — monthly or daily instead of yearly — adds a little more growth.

Why starting early wins

Imagine two savers who both earn 7% a year, compounded monthly. One invests $200 a month from age 25 to 35 and then stops. The other invests $200 a month from 35 to 65. Despite contributing three times as much, the later saver ends up with less: about $244,000 at age 65, compared with about $281,000 for the early saver, whose money simply had more years to compound.

Compound interest on debt

Credit cards compound interest too, usually daily. A $5,000 balance at 24% APR that you never pay down would grow by roughly $1,350 in a year. That’s why paying off high-interest debt is one of the best guaranteed returns available.

How to put compounding to work

You don’t need a large sum to benefit. What matters most is giving your money time and keeping more of its growth.

• Start as early as you can, even with small amounts.
• Automate contributions so they happen every month.
• Reinvest interest and dividends instead of spending them.
• Keep fees low — they compound against you, too.

The bottom line

Compound interest rewards patience. Start early, contribute consistently, keep costs low and avoid carrying high-interest debt, and time will do much of the work for you.

Frequently asked questions