What is the Rule of 72, and how do you calculate it? a An estimate for the number of years it will take for the principal deposit to double, calculated by dividing 72 by the rate of return. b An estimate for the number of years it will take for the rate of return to double, calculated by dividing 72 by the length of time the principal amount is held on deposit. c An estimate for the number of years it will take for an investment to double, calculated by subtracting the rate of return (interest rate) from 72. d An estimate of the time it will take you to graduate from a 4-year college, you divide 72 by the total number of courses you need to graduate.