Finding the area of a pipe's opening in square inches can be useful in determining the flow capacity of the pipe. A pipe with a larger opening allows more fluid to pass through and requires a slower flow rate than a pipe with a smaller opening. You can also use the square inches of a pipe for other calculations, such as determining the pipe's filled weight and finding the surface area inside the pipe.
Use a tape measure to find the width of the pipe opening in inches. Do not include the width of the pipe wall.
Divide the diameter by two to find the inside radius of the pipe opening.
Use this formula to find the number of square inches in your pipe's opening: inside radius x inside radius x 3.1415.