Money & WorkMental model
The mental-math rule that reveals every doubling.
Divide 72 by the annual interest rate in percent and you get roughly the years until your money doubles. At a 6% return it takes 72 ÷ 6 = 12 years; at 2% it takes 36 years. The rule works in reverse too: at 3% inflation, your money's purchasing power halves in about 24 years.
Useful: It lets you compare interest, fees and inflation in seconds without a calculator.
Source: de.wikipedia.org