Measure the Room’s Length and Width
Take the tape measure and measure the length of the room from wall to wall. After recording this measurement on a piece of paper in feet and inches, take a measurement of the room’s width. Write this measurement down on the piece of paper.
If you have a room with an irregular shape, you can cut the room into boxes using a piece of household string. Record the length and width measurements for the separate boxes on a piece of paper.
Perform an Area Calculation
You can calculate the area by taking the rooms length and multiplying it by width. To make it easier, convert your measurement into inches, so that a measurement of 6-feet, 11-inches becomes 83 inches. If your length measurement were 7-feet 5-inches and your width measurement were 5-feet 6-inches, your calculation would be 89 inches times 66 inches, which equals 5,874 inches or around 41 square feet.
Source: www.DoItYourself.com