Compound Interest Calculator

Discover the power of compound interest: calculate how your savings grow exponentially based on initial capital, regular contributions, rate and duration.

Compound Interest Simulator

Initial capital (€)

Monthly contribution (€)

Duration (years)

Annual rate (%)

Compounding

Final capital

68 196 €

Total contributed 34 000 €

Interest earned 34 196 €

With an initial capital of 10 000€ and a monthly contribution of 100€, over 20 year(s) at 5%, you get a capital of 68 196€.

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What is Compound Interest?

Compound interest is often called the "8th wonder of the world" by Albert Einstein. Unlike simple interest, compound interest generates interest on already accumulated interest. It's the snowball effect: your money works for you and generates more and more income over time.

The Mathematical Formula

A = P × (1 + r/n)^nt

A = Final capital (accumulated amount)
P = Initial capital (principal)
r = Annual interest rate (as decimal)
n = Number of times interest is compounded per year
t = Duration in years

The Power of Time

The earlier you start investing, the more compound interest works in your favor. Here's how €10,000 invested at 5% per year grows:

After 10 years

16 289 €

€10,000 invested at 5%

After 20 years

26 533 €

€10,000 invested at 5%

After 30 years

43 219 €

€10,000 invested at 5%

Notice how gains accelerate over time: +€6,289 in the first 10 years, +€10,244 in the next 10, then +€16,686 in the last 10. This is the exponential effect of compound interest!

The Rule of 72

The Rule of 72 is a mental shortcut to estimate how long it takes to double your capital. Simply divide 72 by your annual rate of return.

72 ÷ Rate = Years to double

Doubles in 24 years

Doubles in 14.4 years

Doubles in 10.3 years

10%

Doubles in 7.2 years

Simple vs Compound Interest

Simple Interest

Interest is calculated only on the initial capital. It remains constant each year.

€10,000 at 5% for 5 years

12 500 €

(+2 500 € in interest)

Compound Interest

Interest is calculated on capital + already earned interest. It increases each year.

€10,000 at 5% for 5 years

12 763 €

(+2 763 € in interest)

Difference: +263 € over just 5 years

The gap grows exponentially over time!

Where to invest to benefit from compound interest?

Several investment vehicles allow you to benefit from compound interest. Here are the main ones:

High-Yield Savings Account

No risk, government guaranteed

~3%/year

Life Insurance (guaranteed funds)

Low risk, principal guaranteed

~2-4%/year

Stock Investment Account

Medium risk, long-term horizon

~7-10%/year

World ETF (diversified)

Medium-high risk, highly diversified

~8-10%/year

6 Tips to Maximize Your Compound Interest

Start as early as possible: every year counts, even with small amounts.

Automate your contributions: set up automatic monthly transfers.

Systematically reinvest your gains: don't withdraw interest, let it compound.

Choose accumulating funds: dividends are automatically reinvested.

Minimize fees: 1% less in fees = thousands more euros over 20 years.

Be patient: compound interest rewards the long term, not short-term trading.

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What is compound interest?

Compound interest is the mechanism by which interest generated by an investment is reinvested, itself producing new interest. Albert Einstein reportedly called it the 'eighth wonder of the world': over the long term, its effect is exponential.

Compound interest formula:

Final Capital = Initial Capital × (1 + rate)^years

Example: €10,000 invested at 7% for 30 years → €76,122 (without additional contributions).

The Rule of 72: estimating capital doubling time

The Rule of 72 is a mental shortcut to estimate how many years it takes for your capital to double with compound interest. Simply divide 72 by the annual rate.

annual rate

18 years

to double

Livret A (historical)

annual rate

10 years

to double

World ETF (historical)

10%

annual rate

7.2 years

to double

Growth stocks

The impact of regular contributions (DCA)

Investing regularly, even small amounts, considerably amplifies the compound interest effect. This strategy is called Dollar Cost Averaging (DCA) or scheduled investing. Our simulator lets you include monthly or annual contributions to visualize the impact on your final capital.

Concrete example:

• Initial capital: €5,000
• Monthly contribution: €200/month
• Average annual rate: 7% (World ETF)
• Duration: 20 years
• → Final capital: ~€116,000 for €53,000 invested

Frequently asked questions

Which investments benefit from compound interest?▼

All investments where income is reinvested benefit from compound interest: accumulating ETFs (PEA, securities account), life insurance in unit-linked funds, REITs with dividend reinvestment, and to a lesser extent savings accounts (Livret A, LDDS). Reinvested dividends on individual stocks work the same way.

What is the difference between simple and compound interest?▼

With simple interest, interest is calculated only on the starting capital. With compound interest, the interest generated each period is added to the capital and itself produces interest in subsequent periods. Over 30 years, the difference can be enormous: €10,000 at 7% gives €21,000 with simple interest but €76,000 with compound interest.

What rate of return should I use in the simulator?▼

For index ETFs like MSCI World, the historical annualized return over 30 years is 7 to 10% before inflation. After inflation (2-3%), the real return is 5-7%. For a cautious profile (euro funds, savings accounts), use 2-4%. Past returns do not guarantee future performance.