Rule of 72

A quick shortcut estimating how many years it takes an investment to double at a given rate of return.

The Rule of 72 estimates doubling time by dividing 72 by the annual rate of return: money growing at 8% a year, for instance, roughly doubles in nine years. It's an approximation rather than an exact formula, but it's accurate enough for quick mental math about the long-term power of compound growth.

Using the Rule of 72, she figured her savings growing at 6% a year would roughly double in twelve years.