Compound Interest Calculator

What is the compound interest calculator?

A compound interest calculator projects how a sum of money grows when the interest it earns is itself reinvested to earn further interest. This 'interest on interest' effect is what makes long-term saving powerful. The tool also supports regular contributions, so you can model a realistic savings plan where you add money every month or year on top of your starting balance.

How it works

The growth of the initial principal follows A = P(1 + r/m)^(mt), where P is the starting amount, r is the annual rate (as a decimal), m is the number of compounding periods per year, and t is the number of years. Regular contributions are treated as a series of deposits, each compounding from the moment it is added, using the future-value-of-a-series formula. The tool sums the two components and can show the balance year by year so you can see the curve steepen as compounding accelerates.

Worked example

Start with 10,000 at 6% annual interest compounded monthly for 10 years, adding 200 each month. The initial 10,000 grows to about 18,194 on its own. The monthly contributions total 24,000 but grow to roughly 32,776 thanks to compounding. Combined, the balance reaches about 50,970 — of which nearly 17,000 is interest you never deposited.