From Property Talk Community Wiki

Rule of 72

The rule of 72 is a rule of thumb for calculating how long an amount will take to double in value. This depends on the interest rate or rate of growth (in the case of property values capital growth).

The rule says :

to find the number of years required to double your money at a given interest rate, you just divide the interest rate into 72.

For example, if you want to know how long it will take to double your money at eight percent interest, divide 8 into 72 and get 9 years.