Compound Interest Calculator — Compound interest is interest on interest: your balance grows, and the growth itself grows. This calculator projects a starting amount plus optional monthly contributions over up to 100 years, with yearly, quarterly, monthly, or daily compounding.
Projections are computed locally in your browser.
Projections are simplified models for education, not financial advice. Actual returns depend on markets, fees, and taxes, which this calculator does not model. Talk to a qualified financial professional before investing.
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What is compound interest calculator?
Compound interest is interest on interest: your balance grows, and the growth itself grows. This calculator projects a starting amount plus optional monthly contributions over up to 100 years, with yearly, quarterly, monthly, or daily compounding.
The year-by-year table shows how much of each year’s balance came from contributions versus earned interest — a clear way to see compounding do the heavy lifting over long horizons.
How to use it
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Enter the inputs
Starting amount, annual rate, years, compounding frequency, and monthly contribution.
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Read the summary
Future value, total contributed, and interest earned.
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Review the table
Year-by-year breakdown of contributions, interest, and balance.
Examples
A decade of compounding
$10,000 at 7% compounded monthly with $200/month for 20 years.
P=10000, r=7%, y=20, monthly=200
Result: ≈ $216,000 future value, of which ≈ $158,000 is interest
Frequently asked questions
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Last reviewed: 2026-09-18