The Arithmancy classroom is behind the portrait of a woman with grey hair and a stern expression on the fifth corridor of the North Tower. Students must say the password "Calculations" to gain access to the classroom.

Inside the room are long, dark wooden desks arranged in neat rows. The ceiling is high, and the room is illuminated by the light streaming in through a set of narrow windows.

In addition to the desks, there are several filing cabinets lining the walls, containing scrolls of parchment and old textbooks. Rows of shelves hold various arithmetical tools such as abacuses, astrolabes, and quadrants. A small raised platform at the front of the classroom provides a space for the teacher to stand and deliver lectures, and a large blackboard is positioned next to it for demonstrations.