Ruler! Ruler! is an extremely lightweight free ruler. Ruler! Ruler! dynamically calculates the length of your screen and shows a full screen ruler in both inches and centimeters. It is the most basic and lightweight ruler that you will find for an Android device. And it's free!
Ruler! Ruler! does not have any advertisements on the main ruler page, but it does show a full screen once you're finished with the ruler. Ruler! Ruler! has a wakelock to keep the screen on when you're measuring.
A lot of other free ruler apps have fancy bells and whistles. This is not one of those ruler apps! Ruler! Ruler! is just a simple and free app that turns your device into a ruler.