Break up the room into as many smaller rectangles as necessary, and measure the length and width of each one with the measuring tape. Write each set of dimensions on a piece of paper so you do not forget.
Multiply the length by the width for each rectangle. For example, if the room was divided into 4 rectangles with measurements of 5 feet by 10 feet, 4 feet by 2 feet, 5 feet by 6 feet and 7 feet by 2 feet, the areas would be, in order: 50 square feet, 8 square feet, 30 square feet and 14 square feet.
Add up the all the areas together to get the total floor space area of the room. Using the above example: 50 + 8 + 30 + 14 = 102 square feet.