The Rule of 72

Something that I had never heard of until recently was the rule of 72.  It is a simple technique to help you calculate an interest rate required to double your money in a given number of years - see below

1) calculate the number of years required to double your money at a certain intereste rate - for example -  How long will it take to double your money at 10% interest?  Divide 10 into 72 which equals 7.2 years - so it will take 7.2 years to double your money at 10% interest

2) calculate an interest rate required to double your money over a certain number of years - for example - What kind of interest rate will help double my money in 5 years? Divide 5 into 72 which equals 14.4 - so you need an interest rate of 14.4% to double your money in 5 years