Rule-of-72

This is a very handy "rule" that lets you mentally calculate how long it takes you to double an investment (i.e. compounding), given a particular interest rate. You divide 72 by the interest rate in order to get the number of years. For example, a 7% interest rate would require just over 10 years for an investment to double in value. A 15% interest rate would take between 4 and 5 years to double, and so on.