Sizing of Pipe

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 Sizing of Pipe :

Pipe is identified by three different size categories :
  1. Nominal Pipe Size (NPS)
  2. Outside Diameter (OD)
  3. Inner Diameter (ID)


  • Inside Diameter (ID) is measured from one inside edge of the hoop straight across to the other inside edge.
  • Outside Diameter (OD)  is measured from one outside edge of the hoop across to the other outside edge.
  • Both ID & OD refer to pipe by their actual inside and outside measurments.
  • Nominal Pipe Size (NPS) is used to describe a pipe by name only.
  • "Nominal" refers to pipe in non-specific terms and identifies the diameter of the hole with a non-dimensional number.

  • Based on the NPS and schedule of a pipe, the pipe outside diameter (OD) and wall thickness can be obtained from reference tables which are based on ASME standards B36.10M and B36.19M. 

    • For Pipes having NPS ⅛'' to 12'', the OD is greater than its NPS. For example, the NPS of a pipe is 12'' then OD of that pipe is 12.75".(Note that for tubing, the size indicates actual dimensions, not nominal.)
    • For Pipes having NPS 14" and above, the OD and NPS values are equal. For example, the NPS of a pipe is 14" then OD is also 14". 
***However sometimes the NPS and OD values are not always equal.***

Wall Thickness :

Wall Thickness is a term used to describe the thickness of the metal used to make a pipe.
It is also referred to as a pipe's weight.
They are manufactured in weights known as Standard, Extra Strong & Double Extra Strong.
Schedule number indicates approximate value of expression 1000 P/S where P is the Service Pressure and S is the Allowable Stress both expressed in pounds per square inch (psi)
Schedule is expressed in numbers (5,5S,10,10S,20,20S,.........)
As Wall Thickness of pipe increases the Inside Diameter (ID) of pipe decreases and Outside Diameter (OD) of pipe always remains constant.

***NOTE : NPS is always measured in Inches & Diameter Nominal (DN) is always measured in mm.***

In process piping, the method of sizing pipe maintains a uniform outside diameter while varying the inside diameter. this method achieves the desired strength necessary for pipe to perform its intended function while operating under various conditions.




Click This to Learn how to calculate Wall Thickness of a Pipe.


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