A schedule number indicates the approximate value of the
expression 1000 P/S, where P is the service pressure and S is the allowable stress,
both expressed in pounds per square inch (psi). The higher the schedule number,
the thicker the pipe is. The outside diameter of each pipe size is standardized.
Therefore, a particular nominal pipe size will have a different inside diameter
depending upon the schedule number specified.

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Venugopal