How compounding really works

What compounding means

When your investment earns a return, you don't just earn a return on your starting capital but also on the return you've already accumulated. In the first year, €1,000 at 7% earns €70. In the second year, the 7% is already calculated on €1,070, and so on. At first the difference is small, but over the decades the curve turns sharply upward.

The secret is exponential growth. Linear growth adds the same amount every year; compounding adds a little more every year, because the base has grown. During the first years the difference feels negligible, and that's exactly when many people give up. Most of your final wealth is created in the last years, which is why starting early is decisive.

Time is the most important variable

Compounding rewards time more than amount. Two things decide the end result, how long the money grows and at what return, and of these, time is the one you can influence most reliably. You can't choose your return in advance, but you can choose when you start.

This leads to one of the most important pieces of advice in investing: the best time to start was yesterday, the second best is today. Lost years can't be made up with larger amounts later, and that's exactly what you get to try out with the last tool in this chapter.

Costs compound in reverse

The same force that grows your return also grows the effect of costs, in the wrong direction. Over 30 years, a 1.5% annual fee can eat a quarter or more of your final wealth compared with a 0.2% index fund. Cost is the only variable you can control with certainty: you can't guarantee a return, but you can choose a low cost.

The effect of costs is insidious because it doesn't show up in your account as a separate charge but as a slightly lower return every year. A €100,000 investment at a 7% gross return over 30 years: with a 0.2% fee, the portfolio grows to about €720,000; with a 1.5% fee, only to about €500,000. The difference of more than €200,000 comes from the difference in costs alone, not from a better return, but from the fact that costs didn't get to eat into the power of compounding. Cost is the only variable you can choose in advance and with certainty.

Real return: inflation is the silent opponent

Compounding grows the number of euros, but what matters is purchasing power: how much the money will buy in the future. If your portfolio returns 7% and inflation is 2%, the real return is about 5%. It's the real return that decides whether your wealth actually grows or just keeps pace with inflation. That's why cash is a losing choice over the long term: in nominal terms the amount stays the same, but its purchasing power erodes year by year. Compounding is the most effective way to keep your money ahead of inflation.

Common misconceptions

Compounding is often misunderstood in three ways. First, people think you need a large starting capital. In reality, time makes up for the amount. Second, people expect a steady rise. The real curve swings, and enduring the swings without selling is precisely what it takes for compounding to work. Third, people think it happens automatically. It only works if you don't withdraw the returns along the way. Once you understand these three, compounding turns from an abstract concept into a concrete reason to start early and stay invested.