The Rule of 72, in 60 seconds

The Rule of 72 is the fastest mental math trick in personal finance. Divide 72 by your annual rate of return, and you get the approximate number of years it takes your money to double.

At 8 percent, money doubles roughly every 9 years. At 12 percent, every 6. At 4 percent, every 18.

It is not exact, but it is close enough to make better decisions on the spot, which is the whole point.