Lay down perpendicular to one wall of the room with your feet flat against the wall. You will be using your body as a unit of measurement since you should know your own height.
Determine how many body-lengths it takes to get from one side of the room to the other. Drop an item such as a pen or marker by your head before standing and lying down again so you can accurately keep track of where your previous body-length ended. Multiply the number of body-lengths you count by your height in feet. For example, if you are six feet tall and it takes you four body-lengths to cross a room, your equation will be 6 x 4 = 24. The room is 24 feet in the direction you measured it.
Repeat the body-length procedure with the wall perpendicular to the one you just calculated. Multiply the number of body-lengths along that wall by your height in feet. For example, if it takes you five lengths to get to the opposite wall, your equation will be 6 x 5 = 30. The room is 30 feet in this direction.
Multiply the two calculations by each other to determine the approximate square footage of the room. In the example, 24 x 30 = 720. The area of the room is approximately 720 square feet.