Compound interest gets called "the eighth wonder of the world" so often that the phrase has almost lost meaning. Here's what it actually is, without the hype.
The core idea
Compound interest simply means earning returns on your returns, not just on your original investment. Each period, any growth gets added to your balance - and then future growth is calculated on that new, larger balance.
A simple example
Say you invest €1,000 and it grows by 7% in a year. You now have €1,070. The next year, that same 7% growth applies not just to your original €1,000, but to the full €1,070 - giving you €1,074.90 in growth-on-growth alone, on top of your original amount.
On its own, one year of this doesn't look dramatic. Over 20 or 30 years, it compounds into a very different picture.
Why time matters more than almost anything else
The single biggest input into compound growth isn't the amount you invest - it's time. Money invested earlier has more compounding periods behind it, which is why "start earlier, even with less" is repeated so often in investing education.
Why this connects directly to patience
Compound growth is also part of why long-term, buy-and-hold investing tends to be emphasized so heavily over frequent trading. Every time you sell and sit in cash, you interrupt the compounding process - restarting the clock on however much time it takes for growth to meaningfully build on itself.
The takeaway
Compound interest isn't a trick or a strategy you "activate" - it's simply what happens naturally when you stay invested consistently over time. Understanding it is less about doing something clever, and more about not interrupting a process that already works in your favor the longer you let it run.
Want a quick way to estimate how many years it takes for your money to double at a given return? Our guide on the Rule of 72 covers a simple mental shortcut for exactly that.