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