How to Calculate Wall Area If You Know Floor Area

To paint, wallpaper or panel a wall, you must know the total area of the wall so you can buy the appropriate amount of finish. You can measure the perimeter of a room to calculate the wall area but if you already know the floor area of the room -- from floor plans, for example -- it may be quicker to calculate the wall area based on that number, if your room is square or round. Measure your room from floor to ceiling before beginning the calculation if you don't know the ceiling height.

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Approximate one gallon of paint covers 300 square feet of wall.

Square Room

Step 1

Take the square root of the floor area to get the width of one wall in a square room. If the area is 100 square feet, for example, the width of one wall is 10 feet.

Step 2

Multiply the wall width by four to get the perimeter of the room. For a wall width of 10 feet, the perimeter is 4 times10 feet equals 40 feet.

Step 3

Multiply the perimeter of the room by the ceiling height to get the wall area. If the perimeter is 40 feet and the height is 10 feet, multiply 40 times 10 feet to get a wall area of 400 square feet.

Step 4

Subtract 20 square feet for each standard door and 15 square feet for each standard window from the total wall area if you need the area without doors and windows.

Step 5

Measure the width and height and multiply together for non-standard doors and windows, then subtract the calculated area from the wall area.

Round Room

Step 6

Divide the floor area by 3.14 and take the square root of the result to find the radius of the room. If the area is 100 square feet, for example, the radius is 5.64 feet.

Step 7

Multiply the radius by 6.28 ( 2 pi) to find the perimeter of the room. If the radius is 5.64 feet, the perimeter is 6.28 times 5.64 feet equals 35.42 feet.

Step 8

Multiply the perimeter by the ceiling height to find the wall area. If the perimeter is 35.42 feet and the height is 10 feet, the wall area is 354.2 square feet.

Step 9

Subtract 20 square feet for each standard door and 15 square feet for each standard window from the total wall area if you need the area without doors and windows.

Step 10

Measure the width and height and multiply together for non-standard doors and windows, then subtract the calculated area from the wall area.