Round Pipe
up to 12 inches is measured by the
nominal inside diameter dimensions with
varying wall thickness; round pipe
greater than 12 inches is measured in
the same manner but by the outside
diameter.
Tubing is always measured by the outside diameter dimensions also with varying thickness.
Both pipe and tubing are available in a variety of materials; carbon steel, stainless steel, copper, brass, plastic (pvc & cpvc), in addition to various special alloys (e.g. titanium).

