Definition

The percent change is the percentage a security has changed over a set time period.  

Calculation

Percent Increase  = (Increase/Original Number) X 100

Percent Decrease  = (Decrease/Original Number) X 100

example: Let's say a price drop of $3 on a stock currently trading at $100 is a 3% decrease where as the same $3 a price drop on a stock that is currently trading at $9 is a %30 decrease.

Why is this important?

The percent change helps investors more accurately compare stocks or just see the overall changes an individual stock has experienced over time.  Also, percent change helps accurately see the overall impact of a price increase or decrease over a set moment in time.