To estimate the gallons of paint required for a paint job, divide the total square footage of the surface area to be painted by 350, according to Dummies.com. The number, 350, represents the square footage that a gallon of paint typically covers.
To estimate the amount of paint needed for a 10-foot by 15-foot room with 9-foot ceilings, one door and one window, first figure out the area of the ceiling. Ceiling paint is different from wall paint. The ceiling area would be: 10 x 15 = 150 square feet. Divide this by 350 to get the amount of ceiling paint needed: 150/350 = 0.43 gallons, which rounds up to two quarts.
For the wall paint, measure the area of the four walls. The total of two of the walls would measure: 2 x 10 x 9 = 180 square feet. The other two walls together would measure: 2 x 15 x 9 = 270 square feet. Adding them up yields: 180 + 270 = 450 square feet.
A standard door measures 20 square feet and a standard window is 15 square feet. Subtract these measurements from 450 to get the total paintable surface area: 450 - 20 - 15 = 415 square feet. Divide 415 by 350 to get the number of gallons of paint needed: 415/350 = 1.2. About one gallon and one quart of wall paint would be required. This is only good for one coat of paint, so additional coats require additional paint.