Install screenruler by entering the following commands in the terminal:
sudo apt update sudo apt install screenruler
Description:
measure objects on screen with a variety of metrics
ScreenRuler will display a ruler on screen which allows you to measure the other objects that you've there. It has the following features: * Horizontal and vertical measurement in 6 different metrics: pixels, centimetres, inches, picas, points, and as a percentage of the ruler's length. * Cusomizable colors and fonts. * Keyboard control for precise positioning. * Option for the ruler to stay always on top of other windows.
Homepage: http://gnomecoder.wordpress.com/screenruler/
Version: 0.960+bzr41-1.2
Section: universe/gnome