Learn to measure the height and width of a rectangular shaped object to calculate the square feet. Square footage is a direct measure of the area of an object, or size of its surface. Typical units for width and height, found using a tape measure, include inches and feet. Usually the inch on a tape measure is broken down into 16 equal spaces. A measurement lying at the fourth mark between the 35 and 36 inch lines therefore corresponds to a distance of 35 and 4/16 inches.
Measure the width and height of the object in inches. The height of a rectangular object is the longer side. For example, the width might be 10 and 1/2 inches while the height might be 25 and 3/16 inches.
Convert the fractional part of the width and height measurements into a decimal by dividing the top number by the bottom one on a calculator. Round to two decimal places. Performing this step, for the example, leads to 0.50 inches and 0.19 inches for the fractional parts of the width and height respectively.
Add the fractional part of the width to the width and the fractional part of the height to the height. These are the new values for the width and height. Now, for example, you have a width of 10.50 inches and a height of 25.19 inches.
Multiply the width by the height to obtain the area in square inches. Performing this part of the exercise yields 10.50 inches times 25.19 inches, or an area of 264.5 square inches.
Divide the area by 144 to change to square feet, since a single square foot equals 144 square inches. Completing the example, you arrive at 264.5 square inches divided by 144 square inches per square foot, or an area of 1.8 square feet.