Pipe varies based on its intended use, shape, and size. In their most elemental forms, a pipe is the conveyance of a fluid or gas. They are a couple of exceptions to this primary description, but it mostly stands correct. The intended purpose of pipe help explains the way that the materials are sized.
Pipe sizes, originally, emphasized the importance of the Inside Diameter (ID) as engineers were concerned with the flow of fluid or gas. It was and still is, important that each pipe, of the same size, has a uniform ID to result in a uniform rate of flow. Steel pipe sizes in North America are identified by two non-dimensional numbers: 1) Nominal Pipe Size (NPS), and 2) Pipe Schedule (Sch. or Sched.). For standard pipe, NPS refers to a pipe inside diameter, while the schedule indicates its wall thickness.
The dimensions of steel pipe manufactured in the USA are calculated in inches. In order to ensure conformance across borders, the “inch” identifier has been replaced with Nominal Pipe Size (NPS), a dimensionless designator. It is important to acknowledge that the NPS is a size in name only. The NPS does not always match the actual dimensions of the steel pipe. Rather, steel pipe sizes NPS ⅛ through NPS 12 are identified by their nominal inside diameters. For example, NPS 5 in standard and line pipe has an exact outside dimension of 5.563 inches. Steel pipe sizes 14 and above, on the other hand, are considered “Large OD Pipes” and are identified by their exact outside dimensions.
There are four descriptors for pipe sizes: Nominal Pipe Size (NPS), Outside Diameter (OD), Schedule and Wall Thickness. This is where it gets a little confusing. For pipe sizes ⅛” through 12”, the NPS is LOOSELY based on the ID of the pipe, but they are not the same. For sizes, 14” and up the NPS is equal to the OD of the pipe. Basically, NPS is an antiquated method used to standardize pipe sizes way back when. As the pipe industry evolved, sizes changed.
Today, it is understood that NPS does not equal ID. OD is a more accurate descriptor of the dimension of a pipe. Meaning, if you were to draw a measuring tape across the end of a piece of pipe the reading would be equal to the “OD”. So, OD=OD. Every schedule has a wall thickness, but not all wall thicknesses have a schedule. Schedule and wall thickness are used to define the wall of the pipe. How thick is the wall of the pipe? This is a little tricky, too. Pipes have schedules ranging from 5S to XXH (or XXS). This gets confusing because the actual wall thickness (inches or mm) changes based on the OD of the pipe.
So, 2” Std pipe has a wall thickness of 0.154”, but 3” Std pipe has a wall thickness or 0.216”. So, schedule “standard” can have multiple wall thicknesses based on the pipe OD. There are pipe sizes that have a wall thickness greater than XXH, these are heavier than scheduled pipes. Having said all of that, how is pipe ordered? Well, it comes from steel tube suppliers or steel pipe suppliers and sort of depends on the buyer. The same order can be stated in two different ways:
Option A: 2.000” XH Pipe (NPS x Schedule)
Option B: 2.375” x .218” Pipe (OD x Wall Thickness)
Option A and Option B are equivalent. Steel Pipe is exclusively round (cylindrical) and rigid.
The American National Standards Institute (ANSI) assigned schedule numbers to categorize wall thicknesses based on pressure applications. The ANSI pipe size chart helps pipe customers identify and better understand pipe measurements and determine the ideal steel pipe sizes for their specific project(s). Awareness of pipe specifications is also critical.
There are four dimensions used to measure steel pipe sizes. These dimensions are:
1. Outside Diameter
The Outside Diameter (OD) of a pipe is defined by the distance of a straight line passing through the hollowed center of the pipe, from one outside rim to the opposite outside rim.
2. Inside Diameter
The Inside Diameter (ID) of a pipe is determined by the distance of a straight line passing through the center of a pipe from one inside surface to the opposite inside surface.
3. Wall Thickness
A pipes wall thickness is calculated by subtracting the inside diameter from the outside diameter then dividing the result by two.
4. Weight
The weight of a pipe is determined by its wall thickness. A pipe is manufactured in three weights: 1) standard weight, 2) extra heavy, 3) double extra heavy.
With steel pipe sizes, the weight of a pipe is determined in pounds per foot or kilogram per meter. The weight of a steel pipe in the ANSI pipe size chart is not the actual weight of the pipe. Actual weight differs with the tolerance of varying steel pipe sizes.
The formula for determining the pipe weight per lineal foot is:
W = [OD-WT x WT] X 10.96
W: Weight in Pounds/Foot
OD: Outside Diameter
WT: Wall Thickness
The Outside Diameter and Wall Thickness are measured in inches.
Variations in Steel Pipe Sizes
The industry norm for steel pipe sizes varies by pipe type. For instance, a standard 4.5” OD pipe will be identified as a 4 NPS pipe; meanwhile, a 4.5” line pipe or Oil Country Tubular Goods (OCTG) pipe will be identified as 4.5 NPS pipe. Furthermore, an OCTG pipe is referenced by weight per foot, while a line pipe is typically referenced by wall thickness.
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