Estimate the amount of paint for a room by multiplying the length of the walls by their height and dividing by the number of square feet the manufacturer estimates a gallon of paint covers. Most paints cover about 350 square feet per gallon. Subtract 20 square feet for each door and 15 square feet for each window in the room before dividing.
If the number you calculate has a remainder of less than half a gallon, round the amount down and buy 1 to 2 additional quarts; if it is more than half a gallon, round it to the next full gallon. Check the price of quarts, as it is sometimes more economical to purchase a gallon instead of 2 quarts.
Doors and window trim require a different type of paint than walls do. Estimate the paint for the trim by multiplying the number of doors by 20 square feet and the number of windows by 7.5 square feet. Since you do not paint the glass, the windows subtract more square footage from the wall area than they require for paint. Estimate the square footage of the baseboard by multiplying the perimeter of the room by 0.5. Calculate the area of any cabinets or built-in furniture, and subtract them from the total square footage.
If the walls or trim have never been primed, purchase an equal amount of the appropriate primer for the job. If you plan on using two coats of the finish paint, double the number of gallons you need to purchase.