Today’s Chart of the Day from BlackRock shows a great “thumbnail” rule that finance professionals use to do quick math: The Rule of 72. However, one can also use an even more general rule called The Rule of 7-10. Essentially, if you earn a 7% return, it takes 10 years to double your money, if you earn 10%, it only takes 7 years to double your money. You can even expand it to say if you earn 3% it takes 25 years to double your money, or if you earn 25% it only takes 3 years to double your money.