How long it takes money to double in value. A rough calculation of the time or interest rate needed to double the value of an investment. Divide 72 by the interest rate to determine the number of years it will take money to double. Calculating interest on both principal and previously earned interest. For example: to figure how many years it will take to double a lump sum invested at an annual rate of 8%, divide 72 by 8, for a result of 9 years.