Install kruler by entering the following commands in the terminal:
sudo apt update sudo apt install kruler
Description:
screen ruler
KRuler is a tool for measuring the size, in pixels, of items on the screen. . This package is part of the KDE graphics module.
Homepage: http://www.kde.org/applications/graphics/kruler/
Version: 4:17.12.3-0ubuntu1
Section: universe/graphics