How much should you invest each month?

Regular monthly contributions, 7 % annual return, dividends reinvested. Before tax and before inflation — both corrections come further down.

Take the €300 a month line over thirty years. You pay in €108,000 and end up with €350,836. Interest therefore accounts for 69 % of the final capital: more than two thirds of what you hold at the end, you never paid in.

That proportion is the real answer to the question. It barely depends on the amount — it is time that sets it. Over ten years, interest is 30 % of the total; over thirty, 69 %.

Hence the correct framing: not "how much can I contribute", but "how much can I contribute without ever having to stop". A €150 contribution sustained for twenty-five years comfortably beats €400 abandoned after six, at the first unexpected expense.

Inflation

The €350,836 on the "€300 over thirty years" line is expressed in year-30 euros. With 2 % annual inflation, it represents roughly €194,000 of today's purchasing power. Still a lot, but not the same number — and most simulators forget to say so.

Tax

Of that €350,836, the capital gain is €242,836. Depending on where you live, what remains after tax is:

• Belgium — 10 % above the annual allowance, so well under €24,300 if withdrawals are spread out;
• France, brokerage account — 30 %, about €72,900;
• France, PEA older than five years — 17.2 %, about €41,800;
• Luxembourg — nothing on the capital gain.

The gap between the first and the second-to-last case approaches exceeds €50,000 for an identicaleuro;50,000 for an identical saving effort. The full detail is in our ten-country tax comparison.

The question comes up constantly. At 7 % a year, starting from zero:

• €300 a month — 44 years
• €500 a month — 37 years
• €800 a month — 31 years
• €1,000 a month — 28 years

These durations assume an average return sustained across decades. For reference, the MSCI World returned 8.46 % a year between 1995 and 2025 — while living through eight negative years, including a fall of over 40 % in 2008. Drawdowns are not a theoretical risk: they are part of the road. Our Crises & cycles page lists them one by one.

A generic table gives you the order of magnitude; it does not replace your situation. The calculator takes your starting capital, your contribution, your horizon, your country and your risk profile, then separates clearly what comes from contributions, what comes from interest, what tax takes and what inflation erodes.

Open the calculator

How much does €100 a month grow to in 20 years?

At 7 % a year, €100 a month becomes roughly €50,754 in twenty years, for €24,000 actually paid in. Interest therefore makes up more than half the final capital. Over thirty years, the same contribution reaches about €116,945.

What is the right amount to invest each month?

The amount you can sustain without interruption, once your emergency fund is in place and expensive debt is cleared. Consistency matters more than size: €150 kept up for twenty-five years comfortably beats €400 abandoned after six.

How much do you need to invest monthly to become a millionaire?

At 7 % a year starting from zero, it takes about 44 years at €300 a month, 37 years at €500, 31 years at €800 and 28 years at €1,000. These durations assume an average return sustained across several decades, market falls included.

Is it better to invest a lump sum or monthly?

Statistically, investing an available sum all at once produces a higher average result, because markets rise more often than they fall. Monthly investing reduces the risk of entering just before a fall, and above all it matches the reality of income that arrives every month.

Are these amounts before or after tax?

Before tax and before inflation. Depending on your country of residence, tax takes between 0 % and more than 30 % of the capital gain, and 2 % annual inflation cuts the purchasing power of a sum by roughly 45 % over thirty years. Cumulo's calculator shows both corrections.