Step one: decide your target capital
Before calculating how much to save monthly, you need to know what figure you're saving for. A common way to estimate it is the "4% rule": if you withdraw 4% of your capital each year, that capital tends to last several decades without running out (as a general guideline, not a guarantee). That means that to live on an annual spend of X euros, you'd need roughly 25 times X saved.
For example, if you estimate you'll need €18,000 a year in retirement, the rough target capital would be around €450,000. It's not an exact or universal figure, but it works as a reasonable starting point.
Step two: how many years you have left
The more years until retirement, the lower the required monthly contribution, because compound interest has more time to work. The difference between starting at 30 and starting at 45 isn't just "15 fewer years of contributing": it's 15 fewer years for the first euros contributed to earn interest on interest.
Step three: what return is reasonable to expect
This is where it pays to be cautious. Using a very optimistic rate makes the calculation look "nice" but unrealistic. It's safer to calculate with a conservative scenario and, if the real outcome is better, treat it as extra margin, not as your baseline.
How to calculate it with the tool
In "Calculate goal" mode: enter your target capital, the capital you already have saved, the years until retirement, and a cautious expected return. The result gives you the required monthly contribution, along with a breakdown of how much would be your own contribution and how much would be interest earned along the way.
What to do if the figure comes out very high
- Check whether the target capital is realistic for your case, or whether it can be adjusted with other retirement income sources (public pension, rental income, etc.).
- Consider extending the term if you have margin to do so, even by a few years.
- Start with what you can now and adjust the contribution upward later, rather than not starting at all because you can't cover the ideal figure from month one.