What is this compound interest calculator?
A free, browser-based calculator that projects how money grows when interest compounds over time. Enter a principal, an annual rate, a horizon, and a compounding frequency — and optionally a monthly contribution — to see the total future value, how much of it is your own contributions, and how much is interest earned. Everything runs locally in your browser, so your numbers are never uploaded.
How it is calculated
The future value of the principal uses the standard compound interest formula FV = P × (1 + r/n)^(n·t), where P is the principal, r is the annual rate as a decimal, n is the number of compounding periods per year, and t is years. An optional monthly contribution is compounded monthly at i = r/12 over N = t×12 months as C × ((1+i)^N − 1) / i (or C × N when the rate is zero). Total interest is the total future value minus everything you actually paid in.
- More frequent compounding (monthly, daily) yields a slightly higher future value than annual compounding at the same rate.
- The Rule of 72 — divide 72 by the rate in percent — gives a quick estimate of the doubling time (e.g. 72 ÷ 6 ≈ 12 years).
- Figures are nominal — taxes, fees, and inflation are not deducted.
How to use it
- Enter your starting principal and the annual interest rate.
- Set the number of years and choose how often interest compounds.
- Optionally add a monthly contribution to model regular saving.
- Read the total future value, your contributions, and the interest earned on the right.
Common use cases
- Projecting long-term savings or retirement contributions with regular monthly buys.
- Comparing how different rates or compounding frequencies change the outcome.
- Seeing how much of your final balance is interest versus money you put in.