Frequently Asked Questions

Navigating the Maze of Steel Pipe Sizes: A Guide to Clear Communication

Qualifying the right steel pipe size can be tricky, and miscommunication can lead to costly mistakes. Different systems, terms, and variations exist, making it easy to get lost in translation. This guide will equip you with the knowledge to accurately measure and describe the pipe dimensions.

Understanding Steel Pipe Dimensions

The Three Key Elements

There are three key elements that define a steel pipe:

Outer Diameter (OD):

This is the overall width of the pipe, measured from the outside edge to outside edge. It’s often referred to as the “nominal pipe size” (NPS) or “nominal diameter” (DN).

Wall Thickness (WT):

This is the thickness of the pipe’s metal walls. It plays a significant role in determining the pipe’s strength, pressure capacity, and weight. Different schedules (e.g., Schedule 40, Schedule 80) specify different wall thicknesses for the same OD.

Pipe Length:

This is the overall length of the pipe, typically measured in feet or meters. Standard lengths are usually 20ft (6m) or 40ft (12m), but custom lengths can also be available.

Why These Elements Matter

Knowing these three dimensions is essential for several reasons:

Calculating weight:

With the OD and WT, you can calculate the weight per foot of the pipe, which is crucial for transportation, handling, and structural support considerations.

Determining pressure capacity:

The WT and material properties influence the amount of pressure the pipe can safely handle. Choosing the right pipe for your pressure requirements ensures system integrity and safety.

Estimating cost:

Pipe prices are often based on size and weight, so understanding the dimensions helps you estimate the overall cost of your project.

Choosing the Right Pipe Size

By providing accurate OD, WT, and length specifications, you can ensure you get steel pipes that meet your exact requirements in terms of strength, pressure capacity, weight, and cost. Remember, using the correct pipe size is crucial for optimal performance, safety, and cost-efficiency in your application.

Pipe Dimensions

Steel Pipe Dimension Standards

A Simplified Guide

This section explains the two key standards for steel pipe dimensions: ASME B36.10M and ASME B36.19M.

Key Points:

  • Standards:

    • ASME B36.10M: Covers carbon and alloy steel pipes (welded and seamless) for high/low temperatures and pressures.
    • ASME B36.19M: Covers stainless steel pipes (welded and seamless).
  • Dimensions:

    • Outside Diameter (OD):
      • NPS 12 (DN 300) and above: Nominal size reflects actual diameter.
      • NPS 1/8 (DN 6) to NPS 12 (DN 300): Nominal size based on standard OD, actual diameter slightly larger.
    • Inner Diameter (ID): Varies based on wall thickness.
    • Wall Thickness:
      • Schedules: Standard (STD), Extra Strong (XS), Double Extra Strong (XSS), or Schedule Numbers (5, 10, 20, 30, 40, 80, 120, 160).
      • Schedule “S”: Used for specific thicknesses in B36.19M (NPS 10, 12, 14-22) that differ from B36.10M.
  • Expressing Dimensions:

    • Wall Thickness: Use Schedule (e.g., schedule 40).
    • Diameter: Use Nominal Pipe Size (NPS) or Nominal Diameter (DN).
    • Weight: Use Weight Class (WGT), pounds per foot (LB/FT), or kilograms per meter (KG/M).

Additional Notes:

  • Pipes are used for fluid transmission such as oil, gas, hydrocarbons, water, etc.
  • ASME B36.19M is mostly identical to B36.10M, with few exceptions in specific schedules and sizes.
The Role of Schedule

This text explains what SCH means for steel pipe dimensions and why pipe schedules like 40 and 80 are popular. Here’s a breakdown:

Schedule (SCH):

This number doesn’t directly represent wall thickness, but a series of wall thicknesses for a specific nominal pipe size (NPS).

Key points:

  • Different SCH numbers have different wall thicknesses for the same NPS.
  • Higher SCH numbers generally mean thicker walls for higher pressure resistance.
  • Common schedules include 5, 10, 20, 40, 80, etc. (larger = thicker).

Why SCH 40 & 80 are popular:

  • They balance pressure capacity with material efficiency, making them cost-effective.
  • They’re widely available in various materials like stainless steel (ASTM A312) and carbon steel (API 5L, ASTM A53).

Other key terms:

  • NPS: Standardised size system for pipes, based on inches.
  • DN (Nominal Diameter): Approximate outside diameter (close to inside diameter). Used for interchangeability of pipes and fittings.


  • SCH and NPS work together to specify a pipe’s size and wall thickness.

DN offers a general size reference but doesn’t represent the exact internal diameter.

Nominal Pipe Size (NPS) v’s Nominal Diameter (DN)

Nominal Pipe Size (NPS)

Nominal Pipe Size (NPS) is a US pipe standard for specifying the size of pipes. It is a dimensionless number that refers to the diameter of the pipe in inches. NPS is commonly used in conjunction with Schedule (SCH), which specifies the pipe wall thickness.

Nominal Diameter (DN)

Nominal Diameter (DN) is another way to specify the size of pipes. It is also a dimensionless number, but it is typically measured in millimetres. DN is often used in Europe and other parts of the world.

Relationship between NPS and DN

There is a simple relationship between NPS and DN:

  • DN = 25.4 * NPS (for NPS < 12)
  • DN ≈ NPS * 25 (for NPS ≥ 12)

Key points to remember:

  • NPS and DN are both used to specify the size of pipes.
  • NPS is in inches, while DN is in millimetres.
  • The two can be converted using the formulas above.
  • NPS is more commonly used in North America, while DN is more commonly used in Europe and other parts of the world, but not exclusively.

Here is a table that summarizes the conversion between NPS and DN:

As you can see, the conversion between NPS and DN is not always exact. However, the table above provides a good approximation for most common pipe sizes.


Understanding Pipe Weight Classes:

In the world of pipes, weight class refers to the thickness of the pipe wall. This was originally indicated by three main categories:

  • STD (Standard): This was the original pipe specification, suitable for low-pressure applications.
  • XS (Extra Strong): Developed to handle higher pressure fluids, these pipes have thicker walls than STD pipes.
  • XXS (Double Extra Strong): The thickest of the three, XXS pipes were designed for the highest pressure applications.

However, with advancements in materials processing technology, demand arose for more cost-effective options. This led to the introduction of pipe schedules, a more nuanced system for specifying wall thickness. Each schedule number (e.g., Schedule 40, Schedule 80) corresponds to a specific wall thickness for a given pipe diameter.

The relationship between pipe schedules and weight classes can be found in industry standards like ASME B36.10 and ASME B36.19. These standards provide tables that map each schedule number to its equivalent weight class and other important properties, such as weight per foot and pressure rating.

Key points to remember:

  • Weight class was an older system for indicating pipe wall thickness.
  • Pipe schedules offer a more precise and versatile way to specify wall thickness.
  • ASME B36.10 and ASME B36.19 standards provide the link between schedules and weight classes.

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