Compound interest: the eighth wonder of the world explained with real numbers

What exactly is compound interest?

Simple interest means earning interest on your initial capital. Compound interest means earning interest on the capital plus all the interest accumulated up to that point. The difference looks small at first, but over time it becomes enormous.

A concrete example:

• Simple interest at 8%: €10,000 → grows by €800 every year. After 30 years: €34,000.
• Compound interest at 8%: €10,000 → grows at an accelerating pace. After 30 years: €100,626.

The difference: €66,000 created by compounding that simple interest doesn't produce.

The formula you need to know

The basic calculation is simple:

Final capital = Initial capital × (1 + r)ⁿ

Where r is the annual return (as a decimal) and n the number of years.

At 9% over 30 years: (1 + 0.09)³⁰ = 13.27. Your money multiplies more than 13 times over.

The Rule of 72: calculate it in your head

The Rule of 72 is a brilliant shortcut: divide 72 by the annual return to get the number of years it takes to double your capital.

At 9%, your capital doubles every 8 years. Over 40 years of investing, it doubles 5 times: ×2, ×4, ×8, ×16, ×32.

The real cost of waiting: the numbers that hurt

Suppose you invest €300 a month at 8% annually:

Waiting 10 years (from 25 to 35) costs you more than €500,000, even though you contribute practically the same total amount.

Why are the first years the most valuable?

Because money invested earlier has more time to multiply. €1,000 invested at age 25 at 9% is worth €31,409 by age 65. The same money invested at 45 is worth only €5,604.

Time is literally the most valuable asset you have. Not the amount you invest, not the product you choose: time.

DCA: how to take advantage of compound interest without timing the market

You don't need to invest a large amount all at once. The DCA (Dollar Cost Averaging) strategy involves investing a fixed amount periodically, regardless of whether the market is up or down.

Advantages of DCA:

• Eliminates timing risk: you don't try to guess the best moment
• Automatable: set up a monthly transfer and forget about it
• Psychologically sustainable: during downturns you buy cheaper and avoid panicking

What to invest in: low-cost index funds (Vanguard, iShares, Amundi) tracking the MSCI World or the S&P 500 are the most efficient vehicle for benefiting from compound interest. Fees of 0.07%–0.20% versus 1.5%–2% for actively managed funds.

The difference between a fund with a 0.15% fee and one with a 1.5% fee on €100,000 over 30 years at 8%: more than €150,000 less for the expensive fund.

The impact of inflation

Compound interest works in your favor, but inflation works against you through the same mechanism. With 3% inflation, a capital of €100,000 has the purchasing power of only €41,000 in 30 years.

That's why the return that matters is the real one: nominal return minus inflation. A fund returning 9% with 3% inflation has a real return of about 6%.

Three things that destroy compound interest

1. High fees. A 1.5% fee on €500,000 is €7,500 a year that doesn't get reinvested.

2. Selling during downturns. Getting out of the market in moments of panic and getting back in too late. Historically, missing the 10 best days of the year cuts annualized returns in half.

3. Interrupting contributions. Every month you stop contributing is time your money loses for compounding.

How much do you need to save to have €500,000 by 65?

Try your exact case in our compound interest calculator, where you can simulate different returns, adjust for inflation, and see the impact of fees with side-by-side scenarios.