According to my understanding, Mill Tolerance is the tolerance for variation in the thickness of pipe from nominal pipe thickness which is 12.5% according to B31.3.Feb 4, 2012
Mill Tolerance - ASME (mechanical) Code Issues - Eng-Tips
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ASME B31.3 Pipe Thickness Calculator - Piping Designer
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Jan 16, 2016 - Mill Tolerance - Allowable thickness variation from the mill that produced the pipe. This is defined by the governing specification. For A-53