Money Doubling Calculator (Rule of 72)
The Rule of 72 is the most useful piece of mental maths in investing: divide 72 by your annual return and you get, roughly, the number of years for your money to double. At 12%, that's about six years; at 8%, about nine.
This calculator shows both the shortcut and the exact figure, so you can see how good the rule really is — and how brutally a lower return stretches the wait.
At 10% a year, 100,000 becomes 200,000 in about 7.3 years. The Rule of 72 estimates 7.2 — close enough for mental maths.
How it works
- 01Rule of 72: years to double ≈ 72 ÷ annual return. Fast, and accurate enough for typical rates.
- 02The exact figure uses logarithms: ln(2) ÷ ln(1 + rate). The two barely differ between about 6% and 15%.
- 03The gap between rates is stark — halving your return more than doubles the wait.
Frequently asked
How accurate is the Rule of 72?+
Very, for returns between roughly 6% and 15% — usually within a few months of the exact answer. For very high or very low rates it drifts, which is why the exact figure is shown alongside.
Does it work for inflation and debt too?+
Yes. At 6% inflation, prices double in about 12 years. On a 36% credit card, the debt doubles in two — the same rule, working against you.