Measure the width of the room in feet. Begin at one corner of the room and measure the distance from one corner to the opposite corner.
How to Convert Room Size into Square Feet
There are many instances when it will be necessary to calculate the square footage of a room. If you are planning to change the flooring in the room, you will need to know how many square feet it has. If you are planning to sell your house, the rooms will need to be measured to let prospective buyers know the size of the house and rooms.
Things You'll Need
 Measuring tape
 Calculator
Instructions


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Measure the length of the room just as you measured the width.

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Multiply the length by the width. For example, if the room is 10 feet wide by 12 feet long, multiply 10 by 12. The answer will be 120. The room is 120 square feet.

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Calculate the dimensions differently if the room is not a single rectangle, but shaped like two or more rectangles. For example, a room might have an entry that is 4 by 3 feet, which opens up to a room that is 10 by 12 feet. In this case, multiply the 4 by 3 to arrive at 12 square feet and the 10 by 12 to arrive at 120 square feet.

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Add the two areas together. Add the entry square feet to the room's square feet, which would be 12 + 120. In the example, the room would be 132 square feet.

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Resources
 Photo Credit A. Johnson