What is the ‘Rule Of 72’
The rule of 72 is a shortcut to estimate the number of years required to double your money at a given annual rate of return. The rule states that you divide the rate, expressed as a percentage, into 72:
Years required to double investment = 72 ÷ compound annual interest rate
Note that a compound annual return of 8% is plugged into this equation as 8, not 0.08, giving a result of 9 years (not 900).
BREAKING DOWN ‘Rule Of 72’
The rule of 72 is a useful shortcut, since the equations related to compound interest are too complicated for most people to do without a calculator. To find out exactly how long it would take to double an investment that returns 8% annually, one would have to use this equation:
T = ln(2)/ln(1.08)=9.006
Most people cannot do logarithmic functions in their heads, but they can do 72 ÷ 8 and get almost the same result. Conveniently, 72 is divisible by 2, 3, 4, 6, 8, 9, and 12, making the calculation even simpler.
The rule can also be used to find the amount of time it takes for money’s value to halve due to inflation. If inflation is 6%, then a given amount of money will be worth half as much in 72 ÷ 6 = 12 years. Nor does the unit have to be money: the rule could apply to population, for example.