Compound Interest Calculator
See your future value at your chosen rate — plus, unlike most calculators, a full side-by-side comparison across every rate from 0% to 25%, so you can see exactly how much a rate assumption actually matters.
Your numbers
Growth over time, at your rate
Final value by rate of return
Same principal, same monthly contribution, same number of years — only the rate changes.
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Frequently asked questions
What is compound interest?
Compound interest is interest calculated on both your original principal and the interest that's already accumulated, so your money grows faster over time than with simple interest. Each period's interest gets added to the balance, and the next period's interest is calculated on that larger amount.
How is compound interest calculated?
The standard formula is A = P(1 + r/n)^(nt) — P is the starting principal, r is the annual rate, n is compounding frequency per year, and t is years. Regular contributions each compound separately from the date they're added, which this calculator handles automatically.
What's the difference between simple and compound interest?
Simple interest is calculated only on the original principal, growing by a constant amount each period. Compound interest is calculated on the principal plus all previously earned interest, so growth accelerates over time — and over long periods, the gap becomes dramatic.
How often should interest compound for the best results?
More frequent compounding produces a slightly higher return at the same stated rate, but the difference between monthly and daily compounding is small in practice. The rate itself and the length of time invested matter far more.
What's a realistic annual return to assume?
Broad U.S. stock market index funds have historically averaged roughly 7-10% annually before inflation over long periods, though any single year varies enormously and past performance doesn't guarantee future results. Conservative bond-heavy portfolios typically run lower.