Use the pacing method to roughly calculate the length and width of a room (each step you take is 3 feet). First, multiply the number of steps from one wall to the opposite wall by three. This will give you the approximate width and length of the room. Then multiply the length by the width to get the total room sqft.
For example, a 10 x 10 room would take just over 3 paces from one wall to the opposite wall (3 paces x 3 feet = 9 feet). Round up to get 10 feet for the room's width and length.