Measure every wall of the room by laying the end of the tape measure at one corner and extending it to the next corner. Include doorways and closet doors in the measurement, including these as if they were part of the solid wall.
Write down the measured lengths in feet and inches. For a typical four-sided room, you will have four measurements.
Add up the measurements to obtain the room's linear feet. For instance, if the measurements are 12 feet 6 inches; 10 feet; 12 feet 6 inches and 10 feet, the calculations would be 12 + 10 + 12 + 10 = 44 and 6 + 6 = 12 inches or 1 foot for a total of 45 linear feet.