Compound Interest Calculator

See how savings grow when interest earns interest, with optional monthly deposits.

How this calculator works

Compounding is the reason a modest monthly habit turns into a meaningful balance. Set your starting amount, expected annual return, the number of years and how often interest is added, then add a monthly contribution to see the difference it makes.

Compound interest is the mechanism by which the interest you earn starts earning interest of its own. Over years and decades it is the single biggest reason that regular, modest saving turns into a meaningful balance. The Compound Interest Calculator lets you set a starting amount, an expected annual return, the number of years, and how often interest is added, then optionally layer in a fixed monthly contribution to see the difference consistent saving makes.

The frequency of compounding matters more than people expect. Interest added monthly grows faster than interest added yearly, because each month's interest begins earning its own return sooner. The calculator exposes this so you can compare, say, a savings account that compounds monthly against an investment assumed to compound annually.

Expected returns are assumptions, not promises — investment values go up and down, and past performance does not guarantee future results. Use the tool to understand the shape of compounding and to set realistic savings goals, not to predict a guaranteed outcome. Inflation, taxes and fees will all reduce the real-world value of the final balance.

Everything runs in your browser as you type — nothing is stored or sent to a server. Results are estimates for general guidance and should not replace advice from a qualified professional.

Frequently asked questions

What is compound interest?

Compound interest is interest calculated on the original principal plus all the interest that has already been added. In other words, your interest earns its own interest, which accelerates growth over time.

How does compounding frequency affect the result?

The more often interest is added — monthly rather than yearly, for example — the faster the balance grows, because each addition starts earning its own return sooner.

Should I add a monthly contribution?

Regular contributions have a surprisingly large effect over long periods. Adding even a small monthly deposit can increase the final balance more than a larger one-off starting amount, thanks to compounding.

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