Critical flange dimensions help to identify the standard.
Pipe Size – Pipe flanges also have a corresponding pipe size, generally according to accepted standards.
Outside Diameter of Flange (OD) – this is measured from outer edge to opposing outer edge.
Pitch Circle Diameter (PCD) – this is a diameter that measures from the center of bolt hole to opposing bolt hole.
Flange Thickness – this measures only the thickness of the attaching outer rim, and does not include the part of the flange that holds the pipe.