Chapter 5
The Rule of 72
Want a quick way to see how fast your money can grow? There's a simple trick:
Divide 72 by your yearly return. The answer is roughly how many years it takes for your money to double.
No calculator needed. Just 72 divided by a number.
Examples
A global ETF earns about 8% per year:
72 ÷ 8 = your money doubles every 9 years
The S&P 500 earns about 12% per year:
72 ÷ 12 = your money doubles every 6 years
The Nasdaq 100 earns about 15% per year:
72 ÷ 15 = your money doubles roughly every 5 years
Bitcoin has historically returned ~30% per year:
72 ÷ 30 = your money doubles every 2–3 years (but with wild swings)
Why doubling is so powerful
One double is nice. But the real magic is when your money doubles again and again:
| # of doubles | €10,000 becomes |
|---|---|
| 1x | €20,000 |
| 2x | €40,000 |
| 3x | €80,000 |
| 4x | €160,000 |
| 5x | €320,000 |
Each new double is bigger than all the previous doubles combined. This is why starting early matters so much — every extra year gives your money one more chance to double.
Key takeaway
The rule of 72 makes compound interest easy to picture. At 10% per year, your money doubles every 7 years. Give it 30 years and it doubles over 4 times — turning €10,000 into €170,000+.
Now that you understand why investing works, let's look at what you actually invest in.