Why is the return-on-extra percentage the same on every row?

Because the projection is linear in the contribution. Adding two hundred euros a month produces exactly twice the extra wealth that adding a hundred does, and costs exactly twice the deposits, so the ratio between them cannot change. That is not a flaw in the table, it is the useful part: the multiplier is a property of your horizon and return, so you can read it once and apply it to any increase you are considering.

Is it better to increase now or to increase by more later?

Now, by a margin that grows with the horizon, because the multiplier applies to each contribution for however many years remain after it. An increase starting five years from now misses five years of compounding on every euro it will ever add. Shorten the horizon slider by five years to see the size of that penalty on your own numbers — it is usually larger than the increase you were deferring.

Does this assume I can keep the increase up forever?

It does, and that is the assumption to check hardest. A projection that adds five hundred euros a month for twenty-five years is describing a permanent change in how you live, not a good month. An increase you can actually sustain is worth more than a larger one you abandon in year three, and the page gives you no credit for good intentions.

What another 50 euros a month is worth

Raising a monthly contribution is the one lever in investing that is entirely under your control and entirely unglamorous. This page makes it concrete by running your plan six times — once as it stands, then with five different increases layered on — and reporting not only the extra wealth each produces, but how many euros of extra deposits it took to produce it. That second number is the one that reframes the decision.

The calculator

How it works

Each trajectory is an independent monthly projection at the root of your expected return, with the increase added to the contribution from the first month. Extra wealth is the difference between that final value and the baseline. The cost per thousand column divides the total extra deposits by the extra wealth in thousands, which gives you what a thousand euros of future portfolio costs today. The return-on-extra column is the same relationship as a percentage, and it is identical on every row — the model is linear in the contribution, so doubling the increase doubles the extra wealth and doubles the deposits alongside it. What the rows genuinely differ in is the euro amounts, not the ratio.

What it assumes

A constant return and a flat contribution for the whole horizon, with no inflation adjustment, so the final figures are nominal and the later ones buy less than they appear to. There is no tax and no cost, both of which reduce the extra wealth proportionally rather than changing which row is better. The increase is assumed to start immediately and never to stop, which is the thing most likely to be untrue in practice.

Common questions

Why is the return-on-extra percentage the same on every row?: Because the projection is linear in the contribution. Adding two hundred euros a month produces exactly twice the extra wealth that adding a hundred does, and costs exactly twice the deposits, so the ratio between them cannot change. That is not a flaw in the table, it is the useful part: the multiplier is a property of your horizon and return, so you can read it once and apply it to any increase you are considering.
Is it better to increase now or to increase by more later?: Now, by a margin that grows with the horizon, because the multiplier applies to each contribution for however many years remain after it. An increase starting five years from now misses five years of compounding on every euro it will ever add. Shorten the horizon slider by five years to see the size of that penalty on your own numbers — it is usually larger than the increase you were deferring.
Does this assume I can keep the increase up forever?: It does, and that is the assumption to check hardest. A projection that adds five hundred euros a month for twenty-five years is describing a permanent change in how you live, not a good month. An increase you can actually sustain is worth more than a larger one you abandon in year three, and the page gives you no credit for good intentions.

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What another 50 euros a month is worth