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.
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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.