Rule of 72
Divide 72 by your annual return to estimate how many years it takes money to double. The fastest mental math in finance.
By Findley's Finance · Last reviewed: September 2026
Why it matters
The Rule of 72 turns abstract returns into something you can feel: 8% means doubling every 9 years, 4% means every 18. It also exposes fees and inflation instantly — a 1% fee does not sound like much until the rule shows it steals years of doubling. One division, and compounding stops being a vague concept.
Concrete example
At an 8% average return, 72 ÷ 8 = 9 years to double your money.