We recently posted a list of handy math tricks, and among them is a quick way to estimate how long it will take to double an investment with a given rate of return.
That trick is the rule of 72: Take your interest rate, and divide it into 72. For example: If you expect a 6% average annual return, the doubling time will be 72/6 = 12 years.
That’s much easier than trying to reverse engineer some complicated compound interest formula.