Compound Interest Calculator
See how savings or investments grow with compound interest, including monthly contributions.
How it works
A = P × (1 + r/n)^(n×t) — contributions are compounded month by month
P is the starting amount, r the annual rate as a decimal, n compounding periods per year, t years. Monthly contributions are added at each month’s end and then compound with the balance. More frequent compounding helps, but the rate and time matter far more.
Worked example
$10,000 at 7% compounded monthly for 20 years with $200 added each month grows to about $143,000 — roughly $58,000 of it contributions and $85,000 growth.
Frequently asked questions
What rate should I assume?
A savings account might pay 4–5% today; long-run stock index returns have averaged about 7% after inflation, with large swings year to year. Use a range, not one number.
Does compounding frequency matter much?
Less than people think. $10,000 at 7% for 20 years is $38,697 compounded yearly vs $40,547 daily — the rate and the years dominate.
Is the result adjusted for inflation?
No. To think in today’s money, subtract expected inflation (historically ~2–3%) from the growth rate you enter.
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This is an educational estimate, not financial advice. Lenders use their own rounding, fees and credit terms — confirm exact figures with your lender or a licensed advisor.