72 divided by Interest rate = Years to double investment

For example, if you earned 7% annually on an investment, you would double your original investment in about 10 years:
(72 ÷ 7%) = 10.3 (years)

You can also use it to determine what interest rate you need to double your money in a set amount of years:

72 divided by Years to double investment = Interest rate required

For example, if you wanted to put your money into an investment that would double in value in 7 years, it would have to earn 10% a year.
(72 ÷ 7 (years)) = 10%