Compound interest calculator
Indicative result. Not financial advice.
How compound interest works
With compound interest, the interest you earn is added to your capital and, the following month, earns interest too. Over time the effect speeds up: each year you earn on a larger amount.
This calculator applies the return every month (the annual rate divided by 12) and assumes the monthly contribution is paid in at the end of each month.
Final amount = starting × (1 + i)n + contribution × ((1 + i)n − 1) ÷ i
Where i is the monthly return and n the number of months.
Three examples
- Starting amount only: €10,000 at 5% a year for 20 years grows to about €27,126, with nothing added.
- Contributions only: €200 a month at 5% for 20 years adds up to about €82,207, of which €48,000 is what you put in.
- Both: €5,000 to start and €150 a month at 6% for 15 years gives about €55,893, with €32,000 contributed.
Keep in mind
The return is an assumption: no investment guarantees a fixed percentage, and those that can earn the most can also lose. The result also ignores inflation, fees and taxes, which reduce what you actually keep.
Indicative result, for information only.
Frequently asked questions
What return should I enter?
It is a figure you choose to simulate. Try several (for example 2%, 5% and 7%) to see how the result changes. There is no safe number.
What matters more, contributing more or waiting more years?
Usually time. Starting earlier gives interest more room to earn interest. Change the years and the contribution to see it for yourself.
Why does it not match my bank or fund?
Real products have fees, taxes and returns that change every year. This calculator uses a fixed, constant percentage.