Thank you all! I've solved the problem.

Hi all!

I want to bring the outside world in to the virtual world. I would like to make a highschool CS Map. My highschool.
The problem is that i don't know how to measure the walls in hammer. I'd like to make this map very like my "Torture Chambers", that is my highschool. With the same sizes (the windows, doors, floors)

How do i measure? How do i know that a wall is the same size like my hischool wall. When i go to school i can have a ruler to take all of the proportions... Where is the ruler in hammer?

Thanks