Compound Interest Calculator

Enter a starting amount, an optional monthly contribution, a rate and a number of years — and see the final balance, how much of it you put in, and how much the interest added. The year-by-year table shows the growth curve bending upward, which is the whole point of compounding.

How to use it

Enter the amountsA starting sum, and anything you plan to add monthly. Either can be zero.
Set rate and yearsThe yearly interest rate and how long the money stays put.
Read the splitThe balance is broken into what you contributed and what interest added — and the yearly table shows when compounding starts to dominate.

The curve that pays for patience

Simple interest climbs in a straight line: the same amount added every year. Compound interest curves, because each year's interest joins the balance and earns interest itself. Early on the difference looks trivial — at 5%, the first year on 1,000 is 50 either way. Twenty years on, the compound balance is pulling away decisively, and the interest column in the table is bigger than the contributions column. That crossover is the moment people mean by "your money working for you".

The practical consequence: time in matters more than timing. Ten years at a modest rate beats three years at an impressive one, and starting a monthly contribution early beats a much larger one started late.

Contributions change the shape

A lump sum alone shows compounding at its purest, but most saving is monthly. Contributions dominate the balance in the early years — the table shows "put in" far ahead of interest — and then compounding steadily catches up. With 100 a month at 5%, interest overtakes the annual contribution around year 15. Watching those two columns race is more instructive than any headline number.

Honest assumptions, stated

This calculator compounds monthly and adds contributions at each month's end, and it says so under every result. Real accounts differ: some credit daily, funds compound continuously in effect, and fees quietly subtract. The gaps are small — annual versus monthly compounding at 5% differs by about 0.12% a year — but they exist, which is why the output is a model to reason with, not a quotation.

Your figures never leave your device, and nobody turns them into a sales lead.

Questions people ask

What is compound interest, in one sentence?

Interest paid on interest: each period's growth is added to the balance, so the next period's growth is calculated on a bigger number, which is why the curve bends upward instead of climbing in a straight line.

How is this calculated exactly?

Monthly compounding: the yearly rate is divided by 12 and applied each month, with contributions added at each month's end. Banks and funds vary in their exact crediting schedules, so treat the output as a faithful model rather than a promise of any specific account.

How big is the compounding effect really?

It grows with time, which is the part intuition misses. At 5%, money doubles in about 14 years and quadruples in 28 — the second doubling takes no longer than the first, but it is twice the amount. On long horizons, the interest earned routinely overtakes everything you contributed; the year-by-year table shows the crossover.

What is the rule of 72?

A quick mental estimate: 72 divided by the interest rate gives the years to double. At 6%, about 12 years; at 3%, about 24. It is accurate enough for rates below 10% and useful for sanity-checking any projection, including this one.

Does this account for inflation or tax?

No, and deliberately. Both vary by country and by account type. A common approach for inflation: subtract expected inflation from the rate and read the result as today's purchasing power — 6% growth at 2% inflation is roughly a 4% real rate.

Is this investment advice?

No — it is arithmetic. It shows what a rate produces if it holds; it cannot tell you what rate anything will actually return, or whether an investment suits you. Real returns vary year to year, and averages hide the bumps.