The International Building Code is used for commercial buildings and any building that contains more than two dwelling units, such as apartment buildings. Minimum ceiling height in these buildings must be 7 feet 6 inches in hallways, common areas and habitable rooms.
According the the IRC, all habitable rooms must have a minimum ceiling height of 7 feet. Habitable rooms include bedrooms, living spaces and kitchens but exclude bathrooms, hallways, utility spaces and closets. A ceiling with exposed beams spaced at least 4 feet apart can measure 6 feet 6 inches from the floor to the underside of the beams. Minimum ceiling height in bathrooms is 6 feet 8 inches.
The minimum ceiling height in rooms with sloped ceilings (such as a finished attic space) is 7 feet over at least 50 percent of the room area. Room area is this case is calculated as the total floor space with walls at least 5 feet tall. Areas with ceilings lower than 5 feet are allowed but do not count toward the official room area total (IRC rules also include minimum floor space for most habitable rooms).
Under the IRC, basements with no bedrooms or living areas may have ceilings 6 feet 8 inches high with beams and ductwork at a minimum height of 6 feet 4 inches.