Investing may seem a distant decision or reserved for those who already have large sums of money. But what if we told you that with as little as €100 a month, perseverance and time, you could build a significant sum? The secret lies in compound interest, a financial tool that allows your money to work for you… and the benefits in turn generate more benefits.

In this article we explore in a simple and visual way what happens if you invest that amount month by month. You will see real examples, simulations and comparisons with different interest rates, so you will understand why starting today can make a big difference tomorrow.

What is compound interest?

Compound interest is the process by which the returns generated by your investment are reinvested to generate even more profits. Unlike simple interest, which is applied only on the initial capital, compound interest acts on capital + accumulated interest.

Over time, this effect causes an upward curve in the growth of your investment.

“Compound interest is the most powerful force in the universe,” said Albert Einstein. This is no exaggeration.

Basic simulation assumptions

Let’s see what happens if you invest 100 € every month for 10, 20 or 30 years, applying different annual rates of return.
In all the examples it is assumed that:

You invest €100 on a constant basis each month.
You reinvest the profits obtained.
Profitability remains constant on an annual basis (for illustrative purposes).

Comparative table: how your money grows according to time and profitability

Years of investment	Total contributed (€)	Annual return 4 %.	Annual return 6 %.	Annual return 8 %.
10	12.000	14.729	15.981	17.448
20	24.000	36.596	46.204	58.902
30	36.000	67.267	99.431	135.939

Source: simulations with compounded monthly compounding formula.

What does this table tell us?

Over 10 years, the difference between not investing and earning 8% can add up to more than €5,000 in cumulative gains.
Over 30 years, with an average return of 6%, you could almost triple what you contributed.
And if you reach 8% a year, you would be multiplying your money by almost four times.

This is the power of compound interest: time + consistency + a sustained return = exponential wealth.

Visualizing growth: exponential curve

If we were to graphically represent the 30-year example with an 8% return, we would see a slow curve at the beginning but with an abrupt growth in the last years. This is where the greatest gain is concentrated.

This shows that the earlier you start, the stronger the cumulative effect of compound interest. To lose the early years is to lose a large part of the final growth.

Where to invest with compound interest?

Now, a frequently asked question is:where to invest with compound interest?

Some of the most popular options include:

Index funds or ETFs: with automatic periodic contributions.
Real estate crowdfunding: where the profits obtained can be reinvested in new projects.
Long-term investment plans: that reinvest dividends and returns.
Savings accounts with real compound interest (few, but they exist).

The key is to choose products that allow automatic reinvestment, have low costs and are aligned with your risk profile and time horizon.

What if you can only invest less?

The magic of compound interest does not require large initial amounts. Even with $50 a month you can create significant savings if you start early and are consistent. It’s the habit that matters, not the exact amount.

Conclusion: start today, no matter how small.

The difference between those who start today and those who wait five years is not only what they have contributed in that time, but also what they are missing out on because they have not started the growth cycle.

Investing €100 a month may not seem like much. But applied with intelligence and constancy, it can make the difference between a fair retirement and a comfortable one, or between making ends meet and having financial freedom.

Take the test. Simulate your own scenario. And start building today the wealth you want to have tomorrow.

Simulate your future wealth: What would happen if you invested 100 € every month