Compound Interest Calculator
Enter a starting amount, an optional monthly contribution, the annual interest rate, the number of years and how often interest compounds, then press Calculate. You get the final balance, how much you put in, the interest earned and the balance at the end of every year.
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Formula
Without contributions, A = P × (1 + r/n)^(n·t). With contributions, the balance is grown each month by the equivalent monthly factor (1 + r/n)^(n/12) and the contribution is added at the end of the month.
Limits
Assumes a constant rate, no fees, taxes or inflation, and contributions at the end of each month. Real investment returns vary and are not guaranteed.
Common questions
Does compounding frequency matter much?
Less than the rate and time: at 5%, daily compounding yields only slightly more than yearly. Time in the market dominates.
Can I model withdrawals?
Not yet; contributions must be zero or positive.
Methodology reviewed 2026-10-27. Processing details and test approach.