Place a ladder or stepladder at one corner of the room. Place another ladder or stepladder at the opposite corner. Ask a friend or family member to help you measure.
Have your friend or family member climb the first ladder or stepladder, and hand him the beginning a carpenter's tape measure. Stretching out the tape measure as you go, walk to the second ladder or stepladder, and climb it so that the tape measure is running straight across the room. Record the distance between the opposite corners of the room.
Move the second ladder or stepladder to the other corner directly opposite the first ladder or stepladder. Have your friend or family member stay on the first ladder or stepladder, and once again, give him the beginning of the tape measure. Run the tape measure to the ladder or stepladder that you moved. Climb the ladder or stepladder, and take that measurement.
Multiply the two figures together to determine the square footage of the ceiling. Use this figure when you're ordering your tongue and groove ceiling boards.