To measure the square footage of a rectangular or square space, measure the length and width in feet and then multiple the two numbers together. Thus, to measure a rectangular or square space that is exactly 1 square foot, measure any combination of length and width that multiplies to yield an answer of 1, such as a width and length that are both 1 foot or a length of 2 feet and a width of 1/2 foot.
To determine the square footage of a triangular space, use a measuring instrument such as a ruler or yardstick to measure the base and height of the triangle in feet. Multiply these two numbers, and then divide the result by 2. If the space is in the shape of a right triangle, the base and height are simply the two sides of the triangle that join to form a right angle. For example, a right triangle-shaped space with a 4-foot and a 6-foot side coming together to form a 90-degree angle has an area of (4 feet x 6 feet)/2, or 12 square feet.
If the triangular space does not form a right triangle, select any of the three sides as the base. Measure the length of this base in feet. Determine the height by measuring the length of an imaginary line that runs perpendicularly from this base to the highest point of the triangle. As with the right triangle, divide the product of these two measurements by 2 to determine square footage.