WhatIs.site Compound Interest Calculator
Free compound interest calculator with monthly contributions, multiple compounding frequencies, and a year-by-year breakdown. No signup, all calculations stay in your browser.
Future value
$0
From $0 contributed + $0 in interest.
Contributions vs. interest at end
How compound interest works
Compound interest is the interest you earn on both the original principal and on the interest that has accumulated from previous periods. The longer you let it run, the more dramatic the effect — which is why a typical retirement plan looks like a hockey stick rather than a straight line.
A worked example: invest $1,000 at 7% annual interest, compounded annually. After year 1, you have $1,070. After year 2, $1,144.90 — that extra $4.90 over a simple-interest result is the compound effect. After 30 years you have $7,612. Add a $200 monthly contribution and you end with $254,000. The contributions add up to $72,000 over 30 years; the rest is interest on interest.
Two practical insights this calculator makes obvious:
- Time matters more than rate, in the long run. Doubling your annual contribution from $250 to $500 helps a lot. Starting 10 years earlier helps more.
- Compounding frequency matters less than people think. The difference between monthly and daily compounding at typical rates is small — a few basis points of effective annual yield.
Want the concept first?
If you're new to the math, read the What Is Compound Interest? explainer, then come back to this page to play with the numbers. The interest rate and savings explainers also help with the surrounding concepts.
Frequently asked questions
What is compound interest?
Compound interest is interest earned on both the original principal and the accumulated interest from previous periods. It is the engine behind long-term wealth building: small differences in rate or time produce dramatic differences in outcome.
How is compound interest calculated?
The standard formula is FV = P × (1 + r/n)^(n×t) + PMT × ((((1 + r/n)^(n×t)) − 1) / (r/n)), where P is principal, r is the annual rate (as decimal), n is compounding periods per year, t is years, and PMT is the contribution per period. Our calculator handles the contribution math automatically.
What is the Rule of 72?
A shortcut for compound growth: divide 72 by your annual return rate to estimate how many years it takes for an investment to double. At 7%, money doubles roughly every 10.3 years. At 10%, every 7.2 years. The rule is most accurate for rates between 5% and 12%.
Does the calculator account for monthly contributions?
Yes. You can set any monthly contribution amount. The math converts it to per-compounding-period amounts and supports compounding daily, monthly, quarterly, semi-annually, or annually.
Does this consider taxes or inflation?
No — the calculator computes nominal future value. To estimate real (inflation-adjusted) value, subtract your inflation assumption from your rate of return before calculating. For example, a 7% nominal return with 3% inflation would give roughly 4% real return.
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