The rule of 72
The shortcut: divide 72 by an annual return and you get roughly the years needed to double your money.
- 8% → about 9 years
- 6% → about 12 years
- 12% → about 6 years
Why it is worth memorising: it turns abstract percentages into something you can feel. The difference between 6% and 8% sounds trivial until you see it as doubling every 12 years versus every 9 — over a 36-year career that is three doublings versus four, which is nearly twice the money.
It also works on debt. Credit card debt at 24% doubles in roughly three years if you never pay it.
And on inflation. At 3%, prices double in about 24 years — which is what 'cash is safe' actually costs over a retirement.
Chart: doubling time across return rates.