Using the Photoshop Extended Measurement feature you can measure any area defined with the Ruler tool or with a selection tool, including irregular areas selected with the Lasso, Quick Select, or Magic Wand tools. You can also compute the height, width, area, and perimeter, or track measurements of one image or multiple images. Measurement data is recorded in the Measurement Log panel. You can customize the Measurement Log columns, sort data within columns, and export data from the log to a tab-delimited, Unicode text file.
For a video on measurement features, see www.adobe.com/go/vid0029.