Investment goal calculator — what you need to contribute

Forward projection is a straightforward monthly loop: contribution in, growth at the net monthly rate, per-purchase commission out, custody out as the greater of the percentage and the annual floor divided by twelve, and any lump sum charged its buy commission once at the start. The required contribution is then found by bisection on that same projection, each purchase priced at the contribution being tried. The search doubles an upper bound until the projection overshoots the target, then halves the interval sixty times, which converges to well under a cent, and the answer is rounded up to the next euro so that paying it reaches the target. A closed-form annuity formula cannot be used here because the custody floor makes the monthly deduction depend on the balance, and a commission's minimum makes it depend on the contribution.

The gross return is one fixed rate held for the whole horizon, and the fund's cost is netted off it before the monthly rate is taken. No sale is modelled at the end, because the tool assumes a long hold, so a target reached is a gross balance rather than money in your hand. Inflation is not applied: a target set today in today's euros will buy less by the time you reach it, and the purchasing power page shows how much less.

Why does each broker need a different contribution?

Because the charges are subtracted from the same stream that has to reach the target, so anything taken out has to be replaced by you and then compounded for the remaining years. A few euros of commission per purchase looks negligible next to a five-hundred euro contribution, and over twenty years the replacement cost of those euros is not negligible at all. The column shows the replacement cost directly rather than making you infer it.

Should the target be in today's euros or future euros?

Decide deliberately, because the calculator does not decide for you — it works in nominal terms throughout. If your target is a number you picked because it buys something today, then either raise it by expected inflation over the horizon, or read the answer as buying less than you intended. Twenty years at two and a half percent inflation cuts the purchasing power of a fixed sum by about four tenths.

What happens if the target is unreachable?

The search widens its upper bound until the projection clears the target, so it will always return a number, and for an aggressive target on a short horizon that number will simply be very large. That is the honest answer rather than an error: the required contribution exists, it is just not one you are going to make. Lengthening the horizon moves it far more than raising the assumed return does.

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