Compound Interest Calculator

The compound interest calculator is a free tool that runs entirely in your browser, with no data ever sent to a server. Project a capital with compound interest and regular contributions.

Simple and compound interest

With simple interest, only the starting funds earn: €1,000 at 5 % pays €50 every year, forever. With compound interest, the interest already paid earns in turn: the second year brings €52.50, the third €55.13, and the gap keeps widening. That mechanism is what makes time matter more than the amount.

How much frequency matters

At the same annual rate, monthly compounding pays slightly more than yearly compounding, since interest starts working sooner. The gap stays modest: on 4 % a year, moving from yearly to daily compounding gains roughly 0.08 points of real return.

The rule of 72

To estimate a doubling time in your head, divide 72 by the annual rate. At 6 % it takes about twelve years; at 3 %, about twenty-four. The approximation holds well for rates between 2 and 10 %.

What this tool does not do

This projection is pure arithmetic: it assumes a constant rate and ignores inflation, tax, management fees and market swings. It is not investment advice.

Frequently asked questions

Does the rate account for inflation?

No. It is a nominal rate. To reason in purchasing power, subtract expected inflation from the rate before running the calculation: 5 % of return with 2 % inflation means entering roughly 3 %.

Why does my result differ from my bank’s?

Banks often apply value dates, entry fees and social levies that this calculation ignores. The gap nearly always comes from there, rarely from the formula itself.

Is my data sent to a server?

No: the text you type or paste is processed entirely in your browser, on your device. It is never transmitted to a server.

Do I need an account or an install?

No: the tool works directly on this page, with no sign-up and no installation.