Compound interest calculator

How this compound interest calculator works

The calculator runs month by month rather than year by year, because that is how most people actually save. It takes your annual return, divides it by twelve to get a monthly rate, applies that rate to the balance, and then adds your monthly contribution. The next month starts from the new, larger balance — which is the whole point of compounding: the growth itself starts earning growth.

Three numbers come out of that. The estimated balance is what you would end up with. "Total you put in" is your own money — the starting amount plus every contribution. "Growth from compounding" is the difference between the two. That last figure is the one worth watching, because it shows how much of the result came from time rather than from effort.

What it is useful for

It answers questions like: is my monthly amount enough to reach a goal? Does starting five years earlier really matter? How much difference does one extra percent of return make? Change one input at a time and watch the balance move — you learn far more from that than from any single "correct" answer. It works for a savings account, an index fund, a pension contribution, or any plan where you add money regularly and let it grow.

A worked example

Say you start with $1,000, add $200 every month, assume a 6% annual return and leave it for 15 years. You personally put in $37,000 — the original $1,000 plus 180 monthly payments. The balance ends up around $60,000, so roughly $23,000 of it never came out of your pocket. Now change only the years, from 15 to 25: your contributions rise to $61,000, but the balance climbs past $130,000. You added two-thirds more money and got more than twice the result. That gap is time doing the work.

Two honest caveats. The rate you type is an assumption, not a promise — real markets deliver that average through good years and frightening ones, and the order those years arrive in matters. And the figures ignore inflation, fees and tax, all of which reduce what the money is worth when you finally spend it. Treat the output as a direction of travel, not a forecast.

Before you commit to a long-term plan, make sure the short term is covered with the emergency fund calculator, or see how regular buying works in a volatile market with the crypto DCA calculator.