How it works
Compound interest means earning interest on your interest: each period’s gains are added to the balance, so the next period’s interest is calculated on a larger amount. Over short periods the effect looks minor; over decades it dominates — at 5% annual growth, money doubles roughly every 14 years, and regular monthly contributions amplify the curve dramatically.
Use this compound interest calculator as an investment and savings calculator in one: start with a lump sum, add an optional monthly contribution and an expected annual return, and see the future value with monthly compounding, how much of it you contributed and how much is pure interest — year by year.
How future value is calculated
FV = P × (1 + r)^n + C × ((1 + r)^n − 1) ÷ r
r = AnnualRate ÷ 12
n = Years × 12P is the starting amount, C the monthly contribution, r the monthly rate and n the number of months. The calculator compounds monthly, matching how most savings and investment accounts accrue.