A completely description for steel pipe dimension includes outer diameter (OD), wall thickness (WT), pipe length (Normally 20 ft 6 meter, or 40 ft 12 meters).
  Through these characters we could calculate the pipe weight, how much pressure pipe could bear, and the cost per foot or per meter.
  Therefore, that’s why we always need to know a right pipe size.
  Pipe Schedule Chart unit in mm as below
 
  Steel pipe schedule is a indicating method represented by ASME B 36.10, and also used in many other standards, marked with “Sch”. Sch is the abbreviation of schedule, generally appearing in the American steel pipe standard, which is a prefix of a series number. For example, Sch 80, 80 is a pipe number from chart/table ASME B 36.10.
  “Since the steel pipe main application is to transport the fluids under pressure, so their internal diameter is their critical size. This critical size is taken as nominal bore (NB). Therefore, if steel pipe carry the fluids with pressure, it is very important that pipe shall have enough strength and enough wall thickness. So wall thickness is specified in Schedules, which means the pipe schedule, abbreviated as SCH. Here ASME is the given standard and definition for the pipe schedule.”
  The pipe schedule formula:
  Sch.=P/[ó]t×1000
  P is the Designed pressure, units in MPa;
  [ó]t is Allowable stress of materials under design temperature, Units in MPa.
  If you are new in pipe industry, why you always see a schedule 40 or 80 steel pipe everywhere? What kind of material for these pipes?
  As you have read above articles you know that Schedule 40 or 80 represent a pipe wall thickness, but why it always been searched by buyers?
  Here is the reason:
  Schedule 40 and 80 steel pipe as the common sizes that required in different industries, because of the generally pressure these pipes bear, they are always been asked for a large quantity.
  The material standard for such thickness pipes has no limitations, you could ask sch 40 stainless steel pipe, like ASTM A312 Grade 316L; Or sch 40 carbon steel pipe, such as API 5L, ASTM A53, ASTM A106B, A 179, A252, A333 etc..