The outside of a pipe is always larger than the inside. The difference between the inside diameter, or ID, and the outside diameter, or OD, is caused by the thickness of the walls. By calculating the difference between the inside diameter and the outside diameter it is possible to determine the size, or thickness, of the pipe walls. The process uses basic math and requires no previous plumbing experience or knowledge of pipe work.
Subtract the inside diameter from the outside diameter of the pipe. The result is the combined thickness of the pipe walls on both sides of the pipe. For example, if the outside diameter is 6 inches and the inside diameter is 5 inches, the combined size of the opposite pipe walls is 1 inch.
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Divide the total pipe wall thickness by two. The result is the size, or thickness, of one pipe wall. For example, if the total width of both walls is 1 inch, the size of one wall is 1/2 inch -- 1/2 = 1/2.
Check for errors by reversing the calculations. Double the size of the pipe wall and add it to the internal diameter. If the result is not equal to the outside diameter there was an error in your calculations. Work back through your calculations until the error is resolved.