Free Compound Interest Calculator
Compound interest means you earn returns on your original money plus past returns. Enter your principal, monthly contribution, rate and years to see your projected balance, total contributions and interest earned — all calculated in your browser, nothing uploaded.
| Year | Contributions | Interest earned | Balance |
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How it is calculated
Monthly compounding: each month the balance is multiplied by (1 + annual rate / 12) and then the monthly contribution is added. This equals the closed form A = P(1 + r/12)12t + C × [((1 + r/12)12t − 1) / (r/12)], where P is the principal, C the monthly contribution, r the annual rate and t the years.
Annual compounding: contributions are added each month, and interest is applied once a year to the year-end balance, so a contribution starts earning interest from the following year.
With a 0% rate the balance is simply principal plus all contributions. Years must be whole numbers.
Frequently asked questions
What is compound interest?
Compound interest means you earn returns not only on your original money but also on the returns you have already earned. Over time, that snowball effect is what makes long-term growth much larger than simple interest.
How does this calculator compute the result?
With monthly compounding, each month the balance grows by the monthly rate (annual rate divided by 12) and then the monthly contribution is added. With annual compounding, contributions are added each month and interest is applied once a year to the year-end balance. The exact formulas are shown on the page.
What is the difference between monthly and annual compounding?
Monthly compounding applies interest twelve times a year, so each month's returns start earning returns sooner. Annual compounding applies interest once a year. With the same annual rate, monthly compounding gives a slightly higher final balance.
Does the result include taxes, fees or inflation?
No. The result is a generic illustration that assumes a constant rate and ignores taxes, fees and inflation. It is not financial advice.
Why do small monthly contributions matter so much?
Because every contribution gets years to compound. A modest monthly amount added consistently over decades can contribute more to the final balance than a larger one-off sum added late.