Astm A 53 or A106 water and gas PIPE has a standard outside diameter and wall thickness. It is sold by nominal inside diameter. For example a 1" sch 40 PIPE has an inside diameter of about 1" but the outside diameter is ALWAYS 1.3125".
If you need a 1" schedule 80 PIPE the outside diameter will be the same 1.3125" but the inside diameter will be smaller because the wall thickness is heavier. The wall thickness is determined by using the term "schedule".
A round TUBE 1" is actually 1" outside diameter and you must specify the wall thickness you would like. TUBING is sold by using a gauge number, or by calling out the decimal thickness.
These guidelines are true for all pipes and tubes whether they are Seamless or Welded pipe, Seamless or Welded tubing.