Compound Interest Calculator

How compound interest actually works

Compound interest is what happens when your earnings start earning. In a simple-interest world, $10,000 at 7% pays you $700 every year forever. With compounding, year one pays $700 — but year two pays 7% on $10,700, year three on $11,449, and so on. The gap looks trivial early and enormous later, which is why the curve bends upward instead of running straight.

Each compounding period this calculator does two things, in order: it adds any contribution you're making, then it applies that period's share of the annual rate to the whole balance. Run that loop for enough periods and the balance separates from the money you actually put in — the space between those two lines is interest.

How this calculator gets its answer

There's no trickery to it — it simply walks forward through time, one period at a time. Add your contribution, add that period's growth, move to the next period, repeat until you run out of years. Doing it step by step (rather than with one big formula) is what lets it handle any contribution schedule without the math getting unwieldy.

The part people underestimate: time

Of the three levers — how much you start with, how much you add, and how long you leave it alone — the last one does the heaviest lifting, because it's the exponent. Try it above: take a scenario and add ten years, then instead try doubling the contribution. At stock-like returns the extra decade usually wins — at 7%, roughly $994k versus $866k for doubling the contribution. At low, savings-account rates the extra money wins instead; the higher the return, the more the exponent does the work.

There's a moment worth finding in your own numbers: the year when total interest earned overtakes everything you've contributed. Before it, you're mostly funding the account. After it, the account is mostly funding itself.

Why is a big future balance smaller than it looks?

Because inflation is compounding running against you. At around 3% a year, prices double roughly every 24 years — $100 in 1994 bought about what $220 bought in 2024. A dollar far in the future is a smaller dollar, even though the bill looks the same.

That's why any long-range projection can be shown two ways. "Future dollars" is the raw number — what the account statement would literally say in that year. "Today's dollars" converts it into current buying power — what the money would actually feel like. Saving $500 a month for 40 years at 7% reads about $1.3M in future dollars — and about $400K in today's dollars. Both are true. Only one is easy to reason about, because you think about spending in today's prices.

The practical rule for any calculator, anywhere, including this one: if a projection quotes a giant future balance without saying which kind of dollars it's in, assume future dollars — and mentally shrink it. A 40-year result that doesn't name its currency is flattering you.

The same math has a dark twin. Compounding doesn't care which direction it points: credit-card interest compounds too, which is why high-rate balances balloon so fast. A 20%+ APR is this page's growth curve running against you, on a shorter doubling time than any diversified investment is likely to manage. That symmetry is one reason paying off expensive debt is often described as a guaranteed return — the compounding you stop is just as real as the compounding you start.

None of this changes the headline lesson — time is the biggest lever, and a dollar invested early can outgrow several invested late. It just means reading the output honestly: the growth is real, and so is the shrinkage of the unit it's measured in.

Common questions