iRuler is a simple, very easy to use and beautifully designed pixel Ruler App for your Mac.
It allows you to measure anything on your screen.
How to use:
Just drag and drop the rulers. If you tap anywhere on the screen the rulers will be transparent. Oh, and it has a pointer. Just drag the pointer or tap on the rulers to measure the length in pixels.
Up to six family members will be able to use this app with Family Sharing enabled.