How much can an investment grow if you set aside a sum at the start and then add an amount every month? Thanks to compound interest, far more than you imagine. This investment growth calculator (or savings plan) estimates the final value of your capital after a certain number of years, how much you will have contributed in total, how much the gains are and — often forgotten — how much that figure will really be worth net of inflation. It is the right tool to plan your financial future with numbers.

How an investment grows over time

An investment's growth has two engines. The first is the capital you contribute: the initial amount plus all the monthly contributions. The second, and more powerful in the long run, is compound interest: the gains in turn generate more gains, because each year you also earn on the previous years' earnings. At the start the capital you put in dominates; over the years, the share generated by returns grows until it overtakes it.

How the calculator works

Enter five values: the initial amount, the monthly contribution, the expected average annual return, the number of years and the inflation rate to estimate the real value. The calculator shows the nominal final value, the total you will have contributed, the gains earned, the real value in today's purchasing power and what share of the final result comes from returns alone. All in real time.

The five values you need

The initial amount is what you invest right away; it can also be zero if you start from a pure savings plan. The monthly contribution is the constant sum you add each month. The return is the expected average annual growth: use a realistic value for the type of investment. The years determine the power of compound interest. The inflation serves to translate the future value into today's purchasing power.

Compound interest: the effect that grows

Compound interest grows the capital non-linearly: the more time passes, the more the effect accelerates. Here is an example with €300 a month at 6% a year:

Doubling the years from 20 to 30, the final value more than doubles, while the contributed capital grows only by 50%: the difference is all compound interest.

The power of monthly contributions

Contributing a constant amount every month — the heart of a savings plan — is one of the most effective strategies, because it combines two forces: the accumulation of capital and compound interest on a constantly growing balance. Even modest amounts, contributed consistently over many years, reach surprising results. The calculator makes this clear: try increasing the monthly contribution and you will see the final value rise more than proportionally.

How much the years matter: starting early

Time is the most powerful ingredient of compound interest, more than the return and the contribution. Starting even just a few years earlier can make an enormous difference to the final result, because every extra year multiplies the effect. That is why the most repeated rule in investing is: the best time to start was yesterday, the second best is today. Use the calculator to see how much, in euros, each year of head start is worth.

The return: realistic expectations

The expected return depends on how you invest and the risk you accept. Here are indicative long-term historical references:

These are historical values, not guarantees: real returns vary and can be negative in single years. Use a cautious, long-term estimate, not the result of an exceptional year.

Contributed capital and gains: the share that grows

The calculator shows separately how much you contributed and how much you earned. It is an eye-opening figure: over long horizons, much of the final value is not money you put in, but gains generated by compound interest. The share from returns shows exactly this, as a percentage. Over 20-30 years it can exceed half the total: concrete proof that investing, and doing it for a long time, makes money work in your place.

Nominal value and real value: inflation

A future value of €150,000 looks enormous, but how much will it really be worth in 20 years? Inflation reduces purchasing power over time, so the calculator also shows the real value: how much that future figure would buy at today's prices. It is an essential figure for planning real goals such as retirement or children's education. The goal of investing is precisely to beat inflation and increase purchasing power, not just the number in the account.

The savings plan and risk reduction

Investing a fixed sum at regular intervals, regardless of how the markets are doing, is called a savings plan (or dollar-cost averaging). By buying consistently, you buy more units when prices are low and fewer when they are high, averaging the entry price and reducing the risk of investing everything at the wrong moment. It is a disciplined, automatic strategy, suited to those who do not want (or cannot) time the perfect entry.

Fees: the hidden cost that erodes returns

An often underestimated factor are the fees of the instruments you invest in. A small annual percentage, compounded over decades, can take a huge slice of the final result:

To estimate the effect of fees in the calculator, reduce the expected return by an amount equal to the annual fee: you will see how much they weigh on the result.

Reinvesting the gains

Compound interest only works if the gains stay invested. Reinvesting dividends, coupons and capital gains, instead of withdrawing them, is what fuels the exponential growth. Many accumulating instruments do it automatically. If instead you withdraw the gains, the capital stops growing on itself and the compounding effect switches off. The calculator assumes everything stays invested: the scenario that maximises long-term growth.

How much to contribute for a goal

You can also use the calculator in reverse: if you have a goal (a figure you need in X years), try different monthly contribution combinations until the final value reaches the target. That way you discover how much to set aside each month to get there, given an expected return. It is the most concrete way to turn a financial dream into a plan with precise, sustainable numbers.

Risk and diversification

Higher returns always come with greater risk: markets rise and fall, and the calculator's values are a projection based on a constant average return, not a guarantee. To manage risk, the rule is to diversify across many instruments and keep a long horizon, which allows you to absorb the swings. The shorter your horizon, the more cautious the return you enter should be.

Common mistakes in investing

• Entering an optimistic return based on an exceptional year, not the long-term average.
• Forgetting fees, which reduce the net return.
• Ignoring inflation and confusing nominal value with real purchasing power.
• Withdrawing the gains, switching off compound interest.
• Stopping contributions during downturns, just when you would buy at a discount.

How to use the calculator to plan

Enter your starting situation and try different scenarios: what changes if you increase the contribution by €50 a month? If you start five years earlier? If the return is 5% instead of 7%? By comparing the results you understand which levers matter most in your case. Always look at both the nominal and the real value: the latter tells you how much you will really be able to buy with that figure in the future.

Practical example

You start with €5,000, contribute €300 a month for 20 years at a 6% annual return, with 2% inflation. The result: you will have contributed €77,000 in total, but the final value will be about €155,000: over €78,000 of gains, that is about half the total generated by compound interest. In today's purchasing power, that €155,000 is worth about €104,000. By increasing the contribution or the years, the result grows surprisingly. Enter your numbers for your exact plan.

Updated July 2026