Place the end of your tape measure on the floor, flush with the wall at one end of the room.
Extend the tape measure across the room to the wall at the other end of the room.
Read the measurement in inches while standing directly over the tape measure.
Divide the measurement by 12 to find the linear feet. For example, if the floor is 240 inches long, divide 240 by 12 to get 20 linear feet.