Money doubles in about 72 ÷ (interest rate %) years.
A first-order approximation of compound growth, accurate within ~2% for rates of 4–12%. At 8%, doubling takes ~9 years.
It is a linearisation that is most accurate near 8%. At 1% the true doubling time is 69.7 years and the rule says 72; at 25% the truth is 3.1 years and the rule says 2.9. And it assumes the rate never changes, which no real investment does.
At 8% the rule says 9 yr and the exact answer is 9.006 yr — within half a percent. It holds to within 5% up to 19.2%, then drifts.
72 has more divisors than the mathematically better 69.3, which is the whole reason it won: the arithmetic has to work in your head.
The rule against the exact answer, computed across the range. Inside the shaded band the shortcut is close enough to use; outside it, reach for the calculator.
Estimate with the rule, then check it against the calculator that models it properly.
Open Compound Interest Calculator →Money doubles in about 72 ÷ (interest rate %) years. A first-order approximation of compound growth, accurate within ~2% for rates of 4–12%.