kruler software package provides screen ruler, you can install in your Debian 8 (Jessie) by running the commands given below on the terminal,
$ sudo apt-get update $ sudo apt-get install kruler
kruler is installed in your system.
Make ensure the kruler package were installed using the commands given below,
$ sudo dpkg-query -l | grep kruler *
You will get with kruler package name, version, architecture and description in a table
| Distro | Debian 8 jessie |
| Section | Graphics |
| Package | kruler |
| Version | 4:4.12.3-2 |
| Description | screen ruler |
| Sub Section | Graphics |