Compound interest calculator
Simulate investments with monthly deposits and see year-by-year growth.
Project an investment’s growth: initial deposit, monthly contributions, interest rate (monthly or yearly, with correct compound conversion) and time horizon. See the final amount, how much came from your pocket versus from interest, plus a year-by-year table — the best way to visualize the snowball effect.
The power of compound interest
Compound interest is "interest on interest": each period’s return starts earning in the next period too. It makes an investment grow in a curve, not a straight line — the more time, the more it accelerates. The calculator simulates this growth from a starting amount, monthly deposits and a rate, showing month by month where the money goes.
Monthly deposit and time: the two engines
Two factors drive the result: how much you save each month and how long you let it grow. Early on, deposits matter more than interest; over time, interest overtakes deposits and becomes the bulk of the pile — the famous "snowball effect". The chart separates what you invested from what interest earned, making that turning point clear.
A simulation, not a promise
The rate you enter is fixed in the simulation; real investments fluctuate, and inflation erodes the final amount’s purchasing power. Use the result to compare scenarios and grasp the impact of time, not as a guaranteed return. The calculation runs in the browser. See also the first million and retirement calculators.
Frequently asked questions
How is a yearly rate converted to monthly?
By compound equivalence: monthly = (1 + yearly)^(1/12) − 1. A 12% yearly rate equals ~0.949% per month — not 1% — because interest compounds within the year itself.
Do deposits earn interest from month one?
In this simulation, each monthly deposit enters at the end of the month and starts earning the following month — the conservative standard used by financial simulators. The initial deposit earns from the first month.
Does the calculator account for inflation or taxes?
No — the result is gross and nominal. For a real-terms view, use a "net" rate (subtracting investment taxes) and compare the result with expected inflation over the period.