The rule of 72 is a cool math trick to tell you how many years it will take to double your money if invested at a given interest rate. The rule of 72 works by dividing the interest rate into 72 giving the number of years your money will take to double. For example, if you have your money invested at the rate of 5%, we apply the rule of 72, dividing 72 by 5 giving us the answer of 14.4 years (72/5=14.4). If we invest more aggressively, say at the rate of 8%, the rule of 72 tells us our money will double in 9 years (72/9=9). If we manage to get an annual rate of return of 13%, it will take only 5.5 years for our money to double (72/13=5.5) according to the rule of 72.
This does not take into account taxes. Taxes may in effect reduce our rates of return. If the money in question is in a tax-deferred account, then the formula works fine as is.