A measuring tape. Measure the length and the width along the walls. Multiply those 2. Then, measure the height from floor to ceiling. Multiply the height by the product of length and width.
Calculate the volume by multiplying the measured length and width of the space together, then multiply the result by the height of the room. From the example, 10 * 25 feet = 250 square feet, and 5 * 10 feet = 50 square feet.
