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If you do not have access to the pipe' opening but you know the thickness of the pipe wall, wrap the measuring tape around the pipe to find the circumference in inches. Then divide the circumference by 3.1415 to find the outside diameter. Divide the outside diameter by two to find the outside radius. Then subtract the pipe wall thickness in inches from your result to find the inside radius of the pipe opening.
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.
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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.