The shortcut
Estimated doubling years ≈ 72 / annual rate in percent
At 9%, the shortcut gives 72 / 9 = 8 years. The exact constant-rate formula uses logarithms: ln(2) / ln(1 + r). Monthly compounding changes the exact result slightly.
Useful applications
- sanity-check a sales claim;
- compare the scale of two constant rates;
- explain why retained returns matter;
- catch a spreadsheet error before relying on it.
Where the Rule of 72 fails
It assumes a positive constant return, full reinvestment, and no taxes, costs, withdrawals, or inflation. It does not describe the path of a volatile asset: arithmetic average returns do not reproduce sequence risk. At negative or unusually high rates, the shortcut becomes misleading.
Doubling a nominal balance is also not the same as doubling purchasing power. Fees and inflation lengthen the economically meaningful doubling time.
Test the estimate
In Money Stack, enter principal with no contributions or costs, select Reinvest, and use the horizon from the shortcut. Compare ending capital with twice principal. Then reduce the rate for estimated ongoing costs and run a stress case.
Investor.gov presents the Rule of 72 as an approximate growth tool. It does not endorse an expected return or product.