The bathrooms are typically located toward the back of each floor, by the elevators.
Level 1. From the entrance, head toward the back of the building. They are to the left of the circular staircase.
Level 2. From the entrance, head toward the back of the building. They are to the right near the back elevators.
Level 3. Head toward the back of the building. There are some restrooms on the right before you reach the Jones Room. Additional restrooms are found off the Shatten Gallery (the men's room is before the elevator entrance, and the women's room is past the elevator entrance.
Level 4, Music and Media. Restrooms are to the right of the Service Desk.
Levels 4-8, Stacks. Restrooms are on the other side of the wall you face when you exit the elevators. Walk around the wall to the left.