Basics Rule of 72
Using the Rule of 72 When the Rate of Return is Known
An investor that knows he can earn 12% on his money may ask the question,
"how long will it take to double my money at this rate of return?".
Using our handy Rule of 72, this is a snap to calculate!
Simply divide the magic number (72) by the investor's rate of return (12).
The answer (6) is the number of years it would take to double the investment.
Using the Rule of 72 When the Number of Years is Known
The Rule of 72 can also be used backwards.
An investor that wanted to double his money in a certain number of years
could use the rule to discover the rate of return he would have to earn
to achieve his goal.
A businessman that wanted to double his money in four years, for example,
would divide 72 by four.
The result (18%) is the after-tax compound annual rate of return
he would have to earn to meet his goal on time.
see
http://beginnersinvest.about.com/cs/21jumpstreet/a/012501a.htm