Measure the room. Start by recording the height and width of 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.each wall.Find the total preliminary surface area of the walls by adding the totals of the four walls together. In our example, the preliminary square footage is 700 (150+150+200+200=700). Jot all your figures down with pencil and paper.Account for door and window areas. Subtract the surface area of doors, including the frame, and windows when calculating how much paint you'll need. For example, in room whose wall surface area is 700 square feet, there are 2 doors and a window. 1 door, including frame, measures 4 feet (1.2 m) wide by 8 feet (2.4 m) tall. The other door is 10 feet by 8 feet. The window is 10 feet wide by 4 feet high. The total square footage of these areas is 152 (32+80+40=152).Calculate the net surface area. Subtract the area of the doors and windows from the total surface area of the four walls. In our example, it is 548 square feet (700-152=548).Allow for special circumstances. You might want to paint certain areas, like window sills, the same color as the walls. You can build small areas into your calculations by adding 10 percent to your net surface area. In this example, you would need to add enough paint to account for 54.8 additional square feet of coverage.Account for the doors, door frames and baseboards. Most people paint these items the same color, but choose a different color than they use for the walls.In our example, the surface area of the doors is 112 square feet. The baseboards, if any, usually will be about 3 inches high (7.6 cm) and will extend around the room, minus the space of the door frames. The 2 doors in the example are on the same 20 foot (6.1 m) long wall. One door is 4 feet (1.2 m) wide and the other is 8 feet (2.4 m). That means the baseboard on that wall is 8 feet (2.4 m) long. The other baseboards are 15 feet (4.6 m), 15 (4.6 m) and 20 feet (6.1 m). The total linear footage of the baseboards is 58 feet (17.678 m).Convert the surface area of the 3 inch (7.6 cm) baseboards into square feet. Divide their total linear footage by 4. 58 feet/4=14.5 square feet. To paint the doors, door frames and baseboards in the example, you will need enough paint to cover 126 square feet. Add 20 percent to this figure to account for door frames and touch-ups, bringing the total to 151 square feet.Compute the surface area of the ceiling. Measure the width and length of the floor. Multiply the two. That is the square footage, minus any vents or fixtures. In our example, the ceiling is 200 square feet. Textured ceilings may require slightly more paint.Calculate the amount of paint you'll need to paint the room. Estimates range between 350 to 400 square feet per gallon of paint on smooth, interior plaster walls. In this example, you would need slightly less than 2 gallons of paint to cover the walls with a single coat. If you want to paint doors, door frames, baseboards and the ceiling the same color, you will need an additional gallon of paint. Use these parameters if you're painting with brushes and rollers. If you're using a sprayer, you will need about 10 percent more paint.