The Arithmancy classroom is where Arithmancy lessons are taught; it includes number charts, a blackboard and large rows of tables for students to sit in.

Accessible through a circular trapdoor, it is described as a cross between somebody's attic and an old-fashioned teashop. Small, circular tables are crammed inside it, all surrounded by chintz armchairs and fat little pouffes. The closed curtains and the many lamps draped with dark red scarves create a dim atmosphere softened with crimson light, the scent of burnt tea leaves and incense hangs heavy in the warm air.