This extension measures the dimensions from your mouse pointer up/down and left/right until it hits a border. So if you want to measure distances between elements on a website this is perfect. It doesn't really work with images because there the colors change a lot pixel to pixel.
# Images & HTML Elements
Measure between the following elements: images, input-fields, buttons, videos, gifs, text, icons. You can measure everything you see in the browser.
Your designer handed you mockups as PNGs or JPEGs? Just drop them into Chrome, activate Dimensions and start measuring.
# Keyboard Shortcut
You can start and stop dimensions with the ALT + D shortcut.
# Area Boundaries
Wanna get the radius of a circle? Is text standing in your way? Press Alt to measure the dimensions of a connected area.
# Open Source
Dimensions is Open Source and hosted on Github. Want to port it to Firefox? Go ahead and fork it. Pull requests are welcome! https://github.com/mrflix/dimensions