Compound Interest Calculator
Enter what you're starting with, how much you'll add each month, and the expected annual rate. The year-by-year table shows how much of your balance is your own money and how much is interest.
Compound Interest Calculator
How compound interest works
With simple interest you only earn on your original deposit. With compound interest, each period's interest is added to the balance, and the next period's interest is calculated on that larger amount. Over long periods, the interest you earn on past interest becomes the biggest part of the balance.
A = P × (1 + r/n)n×tExample: $10,000 at 5% compounded monthly for 10 years is 10,000 × (1 + 0.05/12)120 = $16,470.09. Simple interest would give only $15,000.
With regular contributions
Adding money every month usually matters more than the rate. At 6% compounded monthly, $200 a month for 30 years adds $72,000 of your own money and grows to about $200,903. Start 10 years later and contribute for only 20 years: you put in $48,000 and end with about $92,408. The extra 10 years more than doubles the result.
Rule of 72
| Annual rate | Years to double (approx.) |
|---|---|
| 3% | 24 |
| 4% | 18 |
| 6% | 12 |
| 8% | 9 |
| 10% | 7.2 |
| 12% | 6 |
This calculator is for education and planning only. It is not financial advice, and investment returns are not guaranteed.
Frequently asked questions
What is the compound interest formula?
A = P(1 + r/n)^(nt), where P is the starting amount, r is the annual rate as a decimal, n is how many times a year interest compounds, and t is the number of years. $10,000 at 5% compounded monthly for 10 years grows to $16,470.09.
How often is interest compounded?
Savings accounts and CDs usually compound daily or monthly. Bonds often pay twice a year. Stock market returns are usually quoted as an average annual rate, so annual compounding is the closest fit.
What is the Rule of 72?
Divide 72 by the annual interest rate to estimate how many years it takes money to double. At 6% that's about 12 years, and at 9% about 8 years.
What is the difference between APR and APY?
APR is the simple annual rate before compounding. APY includes the effect of compounding over a year. A 5% APR compounded monthly is a 5.12% APY. Banks advertise APY on savings because it's the higher number.
Does this calculator include taxes and inflation?
No. Interest in a regular taxable account is taxed each year, and inflation reduces what the final balance can buy. To estimate real growth, subtract expected inflation (historically about 2–3%) from the rate.
See also
- Amortization Calculator: Monthly payment, payoff date and full schedule, with extra payments.
- Percentage Calculator: Percent of a number, percent change, and increase or decrease by a percent.
- Date Calculator: Days between two dates, or add and subtract days from a date.
Last updated: November 4, 2026