The number of rooms in a building, structure, group of buildings, or other built object is the total quantity of internally divided spaces with a regular usage. Rooms are generally divided from other rooms and spaces by walls or partitions, although in some cases they may be connected without physical separation as long as their distinct identities are abundantly clear.

Rooms include areas used for particular purposes other than transportation. These include living and working spaces, other use spaces, restrooms, lobbies, parking areas, storage areas, waiting areas, and lobbies. Mechanical areas may be included if there is room for human ambulation within them. However corridors, hallways, vestibules, elevator landings, stairways, and other spaces used almost exclusively for internal transportation are generally not counted as rooms.

What counts as a room is set by the source. Some registers count a kitchen and some do not, bathrooms are usually excluded from a room count and always excluded from a bedroom count, and hallways never count.

Because of that, a room count from a real estate listing and one from a building register are rarely the same number for the same building, and neither is wrong.