How to Calculate the Square Foot of a Room

Divide the room into rectangles. Usually a large portion of a room is a rectangle, but sometimes there may be smaller areas when the room is not a perfect rectangle.

Measure two sides of every rectangle in the room. The two sides cannot be parallel sides.

Multiply the two sides of each rectangle to determine the area of the rectangle. Then add the areas of the rectangles. For example, a room has a side measuring 10 feet and a side measuring 12 feet. The room has a small section off the main rectangle that measures 3 feet by 6 feet. The area of the room is 120 square feet (10 feet times 12 feet) plus 18 square feet (3 feet times 6 feet) for a total of 138 square feet.