What Is Compound Interest? A Simple Explanation With Examples
Compound interest is interest earned on interest. Learn how it works, the formula, the Rule of 72, and how to make it work for you instead of against you.
Compound interest is interest you earn on your original money plus the interest it has already earned. Instead of growing in a straight line, your balance grows faster and faster over time, like a snowball rolling downhill.
It’s the single most important idea in saving and investing. It also works in reverse on debt, which is why credit card balances can grow so quickly.
Simple interest vs. compound interest
The easiest way to see the difference is with an example. Say you deposit $10,000 at 5% per year and leave it alone.
| Year | Simple interest balance | Compound interest balance (annual) |
|---|---|---|
| 0 | $10,000 | $10,000 |
| 1 | $10,500 | $10,500 |
| 5 | $12,500 | $12,763 |
| 10 | $15,000 | $16,289 |
| 20 | $20,000 | $26,533 |
| 30 | $25,000 | $43,219 |
With simple interest, you earn $500 every year, always calculated on the original $10,000. With compound interest, each year’s interest is added to the balance, and next year you earn interest on that bigger number. After 30 years, the difference is more than $18,000, without depositing a single extra dollar.
The compound interest formula
The standard formula is:
A = P × (1 + r/n)^(n × t)
- A = the final amount
- P = the principal (your starting deposit)
- r = the annual interest rate, as a decimal (5% = 0.05)
- n = how many times per year interest compounds (12 for monthly, 365 for daily)
- t = the number of years
For $10,000 at 5% compounded monthly for 10 years: 10,000 × (1 + 0.05/12)^(120) ≈ $16,470. That’s about $180 more than annual compounding, because interest starts earning interest sooner.
You don’t need to do this by hand. The calculator at the bottom of this page does it for you, including regular monthly deposits.
The three things that make compound interest powerful
1. Time
Time is the most powerful ingredient, because growth builds on itself. Someone who saves $200 a month from age 25 to 35 and then stops can end up with more at 65 than someone who starts at 35 and saves $200 a month for 30 years, assuming a steady 7% annual return: roughly $260,000 versus $234,000. The early saver contributed $24,000; the late saver contributed $72,000. Starting early matters more than starting big.
2. The interest rate
A higher rate speeds everything up. At 2%, money takes about 36 years to double. At 6%, it takes about 12. Today, the 3-month U.S. Treasury bill rate is 4.09% (as of October 13, 2026), which gives you a sense of what very safe, short-term cash can earn right now.
3. Compounding frequency
Daily compounding beats monthly, which beats annual. But the difference is smaller than most people expect. On $10,000 at 5% for 10 years, daily compounding earns only about $17 more than monthly. That’s why banks advertise the APY (annual percentage yield), which already includes the effect of compounding. Learn more in APR vs. APY.
The Rule of 72: a quick mental shortcut
To estimate how long it takes to double your money, divide 72 by the annual interest rate:
- At 3%: 72 ÷ 3 = about 24 years
- At 6%: 72 ÷ 6 = about 12 years
- At 9%: 72 ÷ 9 = about 8 years
It’s an approximation, but it’s remarkably close for rates between about 2% and 10%.
Compound interest on debt works against you
Credit cards typically compound interest daily on unpaid balances. If you carry a balance at a high APR and only make minimum payments, interest gets added to what you owe, and then you pay interest on that interest. The same math that builds wealth in a savings account can trap you in debt. Paying off high-interest debt is often the best “return” you can get.
Don’t forget inflation
Compound interest grows your number of dollars, but inflation shrinks what each dollar buys. U.S. consumer prices rose 3.7% over the past year (CPI, September 2026). If your savings earn less than inflation, you’re losing buying power even as your balance grows. See How inflation works.
How to put compound interest to work
- Start now, even with a small amount. Time does the heavy lifting.
- Automate monthly contributions so saving happens without thinking about it.
- Reinvest your interest instead of withdrawing it.
- Compare APYs, not just advertised rates, when choosing where to save.
- Pay down high-interest debt first, where compounding works against you.
Key takeaway: Compound interest rewards patience. The earlier you start and the longer you leave your money alone, the harder it works for you.
Try your own numbers below to see how your savings could grow.
Sources
This content is for educational purposes only and isn't financial, tax or investment advice. Rates change daily; confirm current terms with your bank, broker or TreasuryDirect before making decisions. See our full disclaimer.