There are 12 inches in a foot, and there are 12 x 12 square inches in a square foot. Lock that idea in. One square foot equals 12 x 12 square inches. That's 144 square inches in one square foot. An area of 155 square feet contains 144 x 155 square inches, or 22,320 square inches.
1 foot is 12 inches, therefore 1 square foot is (12 x 12) = 144 square inches. To determine the number of square feet, divide the number of square inches by 144 (the number of square inches in a square foot). 1800 / 144 = 12.5 square feet.