Step One. Measure the room. Start by recording the height and width of each wall.
Step Two. Determine the surface area of the walls in square feet or meters. For example, if a wall is 15 feet (4.6 m) wide and 10 feet (3.1 m) high, the square footage of the wall is 150. If the longer walls of the room are 20 feet (6.1 m) wide, the square footage of each wall is 200. Opposite walls in a standard room will have the same surface area.