Compound Interest Calculator
See how compound interest grows your money over time. Enter a starting amount, an optional monthly contribution, your annual interest rate and the number of years to get the future value, the total interest earned, and a year-by-year breakdown.
How to use it
Enter your starting amount, any monthly contribution, the annual interest rate and the time horizon. The calculator compounds monthly and shows your projected balance, how much of it is your own money versus interest, and a chart of the balance climbing over time.
A worked example
Start with $5,000, add $100 a month, and assume a 6% annual return. After 20 years your balance is about $62,755 — but you only ever paid in $29,000. The other $33,755 is interest: more than the money you contributed. That flip, where growth overtakes your own deposits, is the whole point of compounding.
The power of time
Compounding rewards time more than anything else, because interest earns interest and the curve steepens in later years. Here’s that same $5,000 + $100/month at 6% over different horizons:
| Time | You contribute | Interest earned | Final balance |
|---|---|---|---|
| 10 years | $17,000 | $8,485 | $25,485 |
| 20 years | $29,000 | $33,755 | $62,755 |
| 30 years | $41,000 | $89,564 | $130,564 |
| 40 years | $53,000 | $200,936 | $253,936 |
Notice that going from 20 to 40 years roughly quadruples the balance even though you only paid in about $24,000 more. The extra growth is almost entirely interest compounding on interest.
Why starting early matters
Because time is the biggest lever, when you start often beats how much you save. Someone investing $300/month at 7% from age 25 reaches about $787,000 by 65. Waiting until age 35 — just ten years later — lands at about $366,000, less than half, despite contributing only $36,000 less. The decade of extra compounding is worth more than the extra contributions.
Frequently asked questions
How is compound interest calculated?
Each period, interest is added to the balance, and the next period's interest is calculated on that larger balance. This calculator compounds monthly: balance × (1 + rate/12) each month, plus any contribution.
Does it include monthly contributions?
Yes. Enter a monthly contribution and it's added at the end of each month, then compounds alongside your starting amount. Set it to zero to model a single lump sum.
What rate of return should I use?
Use a realistic long-run figure for your account type — savings and CDs are low single digits, while broad stock-market averages have historically been higher but vary year to year. Returns aren't guaranteed; this is an estimate.
Does starting early really matter that much?
Yes — usually more than the amount you save. Investing $300/month from age 25 to 65 at 7% reaches around $787,000, while starting at 35 reaches about $366,000 — less than half, for only $36,000 less contributed. The extra ten years of compounding does the heavy lifting, which is why the best time to start is as early as possible.
Are the results adjusted for inflation or taxes?
No. The figures show nominal growth before inflation and taxes. Inflation reduces what the final balance can buy, and tax treatment depends on the account (a Roth IRA grows tax-free, for example). Treat the numbers as a before-tax estimate and use a slightly lower “real” rate if you want to approximate inflation.
How often should interest compound?
This calculator compounds monthly, which matches how most savings and investment accounts work. More frequent compounding (daily) increases the total slightly; less frequent (annual) decreases it slightly. Over long horizons the difference between monthly and daily is small compared with the effect of the rate and the time invested.
Last updated: July 2026 · How we calculate
BriskToolbox provides estimates for general information only and is not financial advice. Investment returns are not guaranteed.