RULE OF 72

You can quickly figure out in your head how long it will take for a sum of money to double by applying the "Rule of 72."
You simply take whatever interest rate you're earning and divide it into 72. The result will be the number of years required to double your money.
example if the rate of interest is 12% the no of years to double your money will be 72/12 = 6.
If the rate of interest is 9% the no of years to double your money will be 72/9 = 8 years.
You can apply the rule backwards as well.
Let's say you have a lump sum of 5,000 that you would like to grow into 10,000 in 8 years. You would need to find an investment that pays 9% compound interest. (72/8 = 9).
If the best you can find is an 8% return on your money (hypothetically speaking,) then it would take you 9 years to double your money.