Follow the instructions below to calculate how much paint you’ll need. To help, we’ve added an example: A room that is 10 x 15 feet with an 8-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 8 ft. = 400 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: 400 sq. ft. (wall area) – 42 sq. ft. (doors) – 30 sq. ft. (windows) = 328 sq. ft. of walls that need to be painted.
As a rule of thumb, one gallon of quality paint will usually cover 400 square feet. One quart will cover 100 square feet. Because you need to cover 328 square feet in this example, one gallon will be adequate to give one coat of paint to the walls. (Coverage will be affected by the porosity and texture of the surface. In addition, bright colors may require a minimum of two coats.)
Using the rule of thumb for coverage above, you can calculate the quantity of paint needed for the ceiling by multiplying the width of the room by its length: 10 ft. x 15 ft. = 150 sq. ft. This ceiling will require approximately two quarts of paint. (A flat finish is recommended to minimize surface imperfections.)
Painting Doors, Windows, and Trim
The area for the doors and windows has been calculated above. (The windowpane area that does not get painted should allow for enough paint for any trim around doors and windows.) Determine the baseboard trim by taking the perimeter of the room, minus 3 feet per door (3 ft. x 2 = 6 ft.), and multiplying this by the average trim width of your baseboard, which in this example is 6 inches (or 0.5 feet). 50 ft. (perimeter) – 6 ft. = 44 ft. 44 ft. x 0.5 ft. = 22 sq. ft.
Add the area for doors, windows, and baseboard trim. 42 sq. ft. (doors) + 30 sq. ft. (windows) + 22 sq. ft. (baseboard trim) = 94 sq. ft.
One quart will be sufficient to cover the doors, windows, and trim in this example.
Tip: Before you paint, ventilate. Keep a window or outside door open in the room you are painting to avoid breathing noxious fumes.