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To calculate the area of exterior walls, measure the perimeter of the house, or the combined length of all exterior walls. Exclude porches or other “appendages.” Multiply the perimeter by the height of the roofline, or the spot where the top of an exterior wall meets the roof. If your house has gables (peaked parts of the roof), add half the height of a story to the total roofline height to get a ballpark approximation of the area including gables.  Most houses have a height of 9 feet (2.7 m) per story. Add 4.5 feet (1.4 m) to account for any gables. For example, if your house is 24 feet (7.3 m) by 35 feet (11 m) and 2 stories tall, the area in square feet would be (24 + 24 + 35 + 35) x (9 + 9 + 4.5) = 118 x 22.5 = 2,655. To estimate the cost of painting a room, find the total wall area of the room. Measure the perimeter of the room (the combined lengths of all sides of the room) and multiply it by the height of the ceiling.  For example, if the perimeter of your room is 55 feet (17 m) and the ceiling is 10 feet (3.0 m) high, the area of the walls is 550 square feet (51 m2). If the room has any doors and windows, measure the area of each and subtract it from the wall area for a more accurate estimate. Multiply the length of each door or window by its width to find the area. If you’re going to paint your ceilings, you’ll need to factor them in as well. Measure the length of the ceiling and multiply it by the width to get the total area. For example, if the ceiling is 12 feet (3.7 m) by 10 feet (3.0 m), the area is 120 square feet (11 m2). In addition to the walls and ceilings, you may wish to paint your doors and doorframes as well as other trim, such as baseboards. To save the trouble of measuring each of these individually, you can use these tricks to get a good estimate:  Most doors are 21 square feet (2.0 m2), so simply multiply that area by the number of doors you plan to paint. Likewise, the average window is 15 square feet (1.4 m2). While you won’t be painting the windowpanes, the amount of paint you’d need to cover them should also be roughly how much you’d need to cover the window frames and door frames in a given room. To estimate the area of your baseboards, calculate the perimeter of the room and subtract 3 feet (0.91 m) per door. Multiply that by the trim width of your baseboard (e.g., .5 feet (0.15 m). To determine the total square footage you’d like to cover, measure all the surfaces you’d like to paint throughout the entire house, including walls, ceilings, doors, and trim. Add up the results. For example, this might yield a total area of 2,350 square feet (218 m2).
Multiply the house perimeter by the roofline height to measure the exterior. Multiply a room’s perimeter by the ceiling height to find the wall area. Measure the area of the ceilings if you plan to paint them. Measure any doors, windows, and trim you plan to paint. Repeat this process with every room to find the total area.