Rule of 72 Calculator
The Rule of 72 Calculator estimates how long it takes an investment to double at a given annual return — just divide 72 by the return rate. It's a fast mental shortcut for comparing growth rates.
What this means
The rule of 72 estimates doubling time: at 8% return, 72 ÷ 8 = 9 years. The rule works best for rates between about 6% and 10% and is a planning approximation, not exact math.
How we calculate it
Formula
Doubling time (years) = 72 / annual rate.Worked example
An investment returning 9% per year:
- 1Doubling time = 72 / 9 = 8 years
Important assumptions
- The return is compounded annually at a constant rate.
- The rule of 72 is an approximation.
Frequently asked questions
How accurate is the rule of 72?
It's within a few percent of exact doubling time for typical rates between 6% and 10%. Outside that range, the exact compound formula is more accurate.
How can I use this for planning?
It helps compare growth scenarios quickly: a portfolio at 6% doubles in 12 years, while one at 9% doubles in 8 — a meaningful difference over decades.