Step 1
Divide as much as possible of the decking area into rectangles. Rectangles are easy shapes for measuring areas and will simplify your calculations. For example, if the main body of the decking is in an L shape, divide it into two rectangles; if it is C-shaped, divide it into three rectangles. Mark these areas on the decking.
Step 2
Measure the length and width of each rectangle in inches. Multiply the length by the width for each rectangle and record the value. The result is the surface area. For example, an area of decking formed around an L shape may include two rectangles measuring 120 by 72 inches and 108 by 72 inches. Multiply 120 by 72 and 108 by 72 to find their surface areas in square inches. The example results would be 8,640 and 7,776 square inches, respectively.
Step 3
Divide the remaining area into right triangles. Any shape with straight edges can be subdivided into rectangles and triangles. The area of a right triangle is half of the base multiplied the height.
Step 4
Add together the individual areas of all the rectangles and triangles. The combined total is the surface area of the decking in square inches.
For example, an area of decking consisting of two rectangles measuring 120 by 72 inches and 108 by 72 inches and a triangle measuring 36 inches by 48 inches has a surface area of 18,144 square inches -- 8,640 + 7,776 + (1,728 / 2).
Step 5
Divide the total area in square inches by 144, the number of square inches in a square foot. The result is the decking area measured in square feet.
For example, a decking area of 18,144 square inches is equivalent to 126 square feet -- 18,144 / 144 = 126.