Rule of 72 can be used to estimate how long it will take for an investment to double in value. The formula is fairly simple and can certainly give an investor an idea as to whether it would be worth making the investment itself. While this formula can certainly estimate the time to double it seems that there may be other reasons for either making or not making an investment. What reasons do you think might make you select an investment that takes longer to double in value? Does it matter if it earns more interest if it takes longer to double?