how do you calculate the rule of 72 
here s how the rule of 72 works you take the number 72 and divide it by the investment s projected annual return the result is the number of years approximately it ll take for your money to double for example if an investment scheme promises an 8 annual compounded rate of return it will take approximately nine years 72 8 9 to double the invested money note that a compound annual return of 8 is plugged into this equation as 8 and not 0 08 giving a result of nine years and not 900 if it takes nine years to double a 1 000 investment then the investment will grow to 2 000 in year 9 4 000 in year 18 8 000 in year 27 and so on 