The cost is calculated based on the amount of paint used for painting walls.To help you understand we’ve added an example: A room that is 10 x 15 feet with an 9-foot ceiling. The room has two doors and two windows. Measure the total distance (perimeter) around the room. (10 ft. + 15 ft.) x 2 = 50 ft. Multiply the perimeter by the ceiling height to get the total wall area: 50 ft. x 9 ft. = 450 sq. ft. Doors are usually 21 square feet (there are two in this example): 21 sq. ft. x 2 = 42 sq. ft. Windows average 15 square feet (there are two in this example): 15 sq. ft. x 2 = 30 sq. ft. Take the total wall area and subtract the area for the doors and windows to get the wall surface to be painted: 450 sq. ft. (wall area) – 42 sq. ft. (doors) – 30 sq. ft. (windows) = 378 sq. ft. of walls that needs to be painted. You can calculate the quantity of ceiling paint needed for the ceiling by multiplying the width of the room by its length: 10 ft. x 15 ft. = 150 sq. ft. So in this example total paint surface is 528 sq. ft.