Compound Interest Calculator
A compound interest calculator shows how a starting amount plus regular contributions grow when each period earns interest on the whole balance, including past interest. Set your starting amount, monthly contribution, rate, and years to see the final balance, a growth chart, and exactly how much of the total is interest. Runs in your browser, with no signup.
Updated for 2026Your plan
Year by year growth
This table shows the balance at the end of each year, split into what you put in versus the interest earned. Notice how the yearly interest grows even when your contribution stays the same, because it is earned on a larger balance every year.
| Year | Contributed | Interest earned | Balance |
|---|
How compound interest is calculated
For a single starting amount, the future value after n periods is:
where P is the starting amount, i is the rate per period, and n is the number of periods. When you also add a fixed contribution each period, the contributions form an annuity and add:
This calculator compounds monthly, so i is the annual rate divided by 12 and n is the number of years times 12. Contributions are added at the end of each month.
- Starting amount (P)$10,000
- Monthly contribution (PMT)$500
- Annual return (7 percent, so i)0.0058333
- Periods (10 years, so n)120
- Total contributed$70,000
- Total interest$36,639
- Final balance$106,639
Frequently asked questions
What is compound interest?
How much do monthly contributions matter?
What return rate should I use?
Does this calculator store my numbers?
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