Compound Interest Calculator
Calculate how your investments grow over time with compound interest.
Future Balance
Total Principal
Total Interest
Yearly Breakdown
| Year | Balance | Interest Earned | Total Contributions |
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What this calculator does
This calculator shows how an investment grows when the interest it earns is added back and starts earning interest too. Enter a starting amount, a monthly contribution, an annual interest rate and a number of years. You'll see the final balance, how much of it you paid in, how much came from interest, and a year-by-year table.
How to use it
- Initial Investment: the amount you start with. Commas are added as you type.
- Monthly Contribution: what you add each month. Enter 0 to see a single lump sum grow on its own.
- Interest Rate (Annual %): the yearly rate you expect to earn, for example 7.
- Years to Grow: how long the money stays invested.
- Select Calculate Growth to see the totals and the yearly breakdown.
How compound interest works
Compound interest is interest earned on both the money you save and the interest you've already earned. The US Consumer Financial Protection Bureau's example: $1,000 at 5% earns $50 in the first year. In the second year it earns $52.50, because the $50 of interest is now earning interest too. Over long periods that snowball effect does most of the work.
The formula
This calculator compounds monthly: each month, the balance earns one twelfth of the annual rate, then that month's contribution is added. That is the same as this formula, where P is the starting amount, PMT the monthly contribution, r the annual rate as a decimal, and t the number of years:
FV = P × (1 + r/12)12t + PMT × [(1 + r/12)12t − 1] ÷ (r/12)
A worked example
With the default values, $5,000 invested at 7% a year with $200 added every month for 10 years:
- Future balance: $44,665
- Total you paid in: $29,000 ($5,000 + $200 × 120 months)
- Total interest earned: $15,665
More than a third of the final balance is interest. Leave the same plan running for 20 years instead of 10 and the interest overtakes what you paid in.
What to keep in mind
- Rates aren't guaranteed. Savings accounts and bonds pay roughly fixed rates. Stock market returns vary year to year and can be negative. A steady 7% is a simplification for planning.
- Inflation. The result is in future dollars, which buy less than today's. Subtract expected inflation from your rate to see growth in today's money.
- Fees and taxes reduce real-world returns, and this calculator doesn't include them.
- Compounding frequency matters a little. Banks may compound daily or yearly; this tool uses monthly, which matches monthly contributions.
Frequently asked questions
What's the difference between simple and compound interest?
Simple interest is paid only on the original amount. Compound interest is also paid on interest already earned, so the balance grows faster every year.
How long does it take to double my money?
A quick estimate is the "rule of 72": divide 72 by the annual rate. At 7%, money roughly doubles in about 10 years (72 ÷ 7 ≈ 10.3).
Does starting earlier really matter?
Yes. Time is the strongest factor in the formula, because each year's growth builds on all the years before it. The same monthly amount started ten years earlier can end up worth far more.
What rate should I use?
Use the rate your account actually pays, or a conservative long-term estimate for investments. Try a few rates to see a range of outcomes rather than relying on one number.
Is this financial advice?
No. It's a planning estimate. For decisions about your savings or retirement, talk to a qualified financial professional.
Sources: Consumer Financial Protection Bureau, How does compound interest work? Last reviewed on SuiteWebTools: October 2026.