About the Rule of 72 Calculator
The Rule of 72 is a famous mental shortcut: divide 72 by an annual return to estimate how many years it takes to double your money. This calculator shows that estimate alongside the exact doubling time so you can see how close the shortcut is.
How it’s calculated
Years to double ≈ 72 ÷ annual return (as a percent). The exact figure is ln(2) ÷ ln(1 + rate).
Example calculation
At an 8% return, the Rule of 72 estimates doubling in 9 years — the exact figure is about 9.0 years.
Frequently asked questions
How accurate is the Rule of 72?
Very close for typical returns (roughly 4–15%). It's a mental estimate, not a precise formula, but it's remarkably handy.
Can I use it for inflation?
Yes — divide 72 by the inflation rate to estimate how long it takes prices to double, or your purchasing power to halve.
Projections are illustrative and assume a constant rate of return; real investments fluctuate and may lose value. Past performance does not guarantee future results. This is not investment advice.