From overall pipe diameter and wall thickness to pipe lengths, there are several aspects of pipe dimensions. They are invaluable in pipe purchasing, from helping choose the highest pipe schedule for a diameter range to ensuring that the outer diameter is precise enough.

Subject to pipe specifications and customizations, these factors ensure that buyers get the best product that fits their application.

Nominal Pipe Size (NPS)

NPS or Nominal Pipe Size is a term/number used to denote a pipe's size. Rather than referring to the outside or the inside diameter, NPS is a general diameter guideline for the tubing.

It alludes to the bore or the internal flow of the pipe. As a metric/European equivalent to the American imperial measure, Nominal Diameter is the same as Nominal Pipe Size.

This diameter guideline was introduced by the Iron Pipe System of pipe measurements in the early, late 19th to early 20th centuries. This system was then and then taken over by the American Society of Mechanical Engineers (ASTM) in the 1930s, which introduced the following schedules/wall thicknesses to the existing sizes

  • Schedules- 160, 140, 120, 100, 80, 60, 40, 30, 20, 20, 5 

This wall thickness or schedule is one of the factors influencing the NPS, especially for pipes sizes below NPS 14. Since most of the sizing for the range is based on fixed Outside Diameter (OD), the Inside Diameter (ID) also decreases with a rise in wall thickness, ensuring that the NPS falls somewhere between the two.

In fact, for pipes below NPS 12, the OD is somewhat bigger than the size marker, while for sizes NPS 14 and above, these numbers match up exactly. For example

Nominal Pipe Size (Inch)Outer Diameter (Inch)
14 14 
  • Note: NPS=OD for NPS above 14 to NPS 48
Pipe dimensions

Nominal Pipe Size (NPS): Key Points To Remember

  • Though Nominal Pipe Size refers to general pipe diameter, for sizes above NPS 14, the Outside Diameter (OD) and the NPS are the same.
  • Pipes with a nominal sizing less than NPS 12 will have a higher outside diameter than the specified NPS. For example, the outside diameter of an NPS 5 pipe will have an OD greater than 5 inches.
  • The fixed Outside Diameter (OD) greatly influences the lower NPS, specifically NPS ⅛ to NPS 12, with the Inside Diameter (ID) decreasing with an increase in pipe schedule. 
  • For pipe sizes smaller than NPS 14, the Nominal Pipe Size is calculated by considering both the Outside Diameter (OD) and the Inside Diameter (ID).

Wall Thickness (Schedule)

Schedule or wall thickness is an important component of a pipe's dimensions. Usually measured in inches or millimeters, SCH or schedule is less a fixed scale of measurement and more a general numbering system based on a specific formula. 

Importance of Pipe Schedule 

With a professional number-based guide, customers can choose the most durable pipes for corrosive, high-pressure applications within the same diameter range. This guide number is also called the Schedule Number and is vital for customers looking for pipes with higher pressure resistance.

Even pipes with similar NPS can have different Schedule Numbers, meaning that though they have the same diameter, they have varying wall thicknesses. Usually, the wall thickness at a given NPS directly affects the pipe's pressure resistance, but stainless steel; pipes are an exception to this rule.

Even with their lower schedules, this variant of steel pipes can withstand higher pressure and corrosion when compared to their high-schedule carbon alloy steel counterparts.

How Is Schedule Number Calculated?

Each pipe variety has a fixed wall thickness at a given NPS that is mentioned via its schedule number. The Schedule number for each pipe refers to its ability to withstand a specific amount of pressure.

This figure is fixed by the American Society of Mechanical Engineers ASME. It is generated by dividing the material's service pressure by its allowable stress, which is unchanging for a specific temperature. 

The Schedule Number is an expression of this approximate value against X 1000 in pounds per sq inch measurements. In simple terms, the formula for getting a pipe's Schedule Number can be expressed as 

  • Schedule Number = Service Pressure (psi)/ Allowable Stress (psi) X 1000 

As a result of such a calculation, a material's service pressure highly influences its schedule, as its increase will also reflect a rise in the pipe's wall thickness.

This equation is accurate and based on a variation of Barlow's Wall Thickness Formula, which calculates the relationship between a pipe's nominal thickness, diameter, allowable pressure, and internal stress.

As per this formula, the pipe schedule numbers under ASME B36.10 specfications include;

  • ASME B36.10 : STD, XD, XS, XXS, 160, 140, 120, 100. 80, 60, 40, 30, 20, 20, 5

Pipe Schedule For Stainless Steel Pipes

This summary is a standard schedule specification for only carbon alloy pipes. For the scheduled number of stainless steel and nickel alloy tubing, you can refer to the specs under

  • ASME B36.19: 5S, the 40S, 10S, and 80S

Due to its corrosion-resistant properties, stainless steel pipes are preferred over carbon steel alloys. Welded stainless steel pipes with high schedules can work as effective competitors to seamless pipes because of advanced ERW welding methods. 

However, these properties make stainless steel pipes more costly than carbon steel allies. To combat this problem, the ASME has allowed stainless steel pipes of various schedules under ASME B36.19 specifications.

These pipes can be recognized by the "S" accompanying their schedule number, as seen in 5S, 10S, the 80S, and 40S pipes.

Important Points To Remember About Pipe Schedule

  • The standard schedule for carbon alloy pipes is dictated by ASME 36.10 specifications, while ASME B36.19 covers the schedules of stainless steel pipes.
  • The wall thickness of pipes is calculated by using a modification of Barlow's Wall Thickness Formula, which includes internal stress, nominal thickness, and more as factors.
  • The ASME usually oversees the standard wall thickness for pipes.
  • Despite having lower schedules, stainless steel pipes are more pressure and corrosion-resistant than carbon alloy steel pipes.
  • The wall thickness of stainless steel pipes is expressed via an "S" attached to their schedule number.
  • A higher schedule number is associated with increased pressure resistance.

Pipe Length

Tubing/ pipe lengths generally conform to a general sizing chart expressed in metric or imperial measurements. However, pipe manufacturing is uneven, producing pieces of varying lengths.

Keeping these factors in mind, the two chief lengths assigned to pipes, i.e., Single Random Length and Double Random Length, aim to address and cover these issues in size. 

The term random is used in Double Random Length (DRL) and Single Random Length (SRL) because of the variables in pipe production. During manufacturing, the pipe mills can control the maximum and minimum lengths of the tubing being processed. 

But since it cannot control the dimensions of every single piece, the pipe is produced in random sizes. This "randomness" of the process applied for each piece gives DRL and SRL pipes their name.

And while they do fall under the maximum-minimum requirements, these pipes are ultimately subject to the production variables.

On the other hand, cut lengths are purely made-to-fit pipes created under specific instructions. These pipes are made according to the customer's demands, who usually order these lengths for projects which require non-standard sizing.

Single Random Length

SRL or Single Random Length pipes are shorter than Double Random Length pies, with average lengths varying between 17 feet to 24 feet. They are below 4 to 2 inches in diameter and have varied pipe endings, including threaded, plain, and more.

As with DRL, pipe specifications influence SRL pipe lengths, with about 5% of all pipes falling under the SRL average by measuring somewhere between 12 feet to 12 feet.

Characteristic Features Of Single Random Length (SRL) Pipes

  • Single Random Length sizing covers any random pipe below 4 inches in diameter.
  • SRL pipe lengths usually refer to any random pipe between 16.4 feet to 23 feet. However, 5% of all SRL pipes include tubing of 12 feet to 15.7 feet.
  • SRL pipes generally get made using half measures of larger bore pipes.
  • Single Random Length pipes are typically plain-ended within the 18 to 15 feet range and threaded and coupled in the 18 to 22 feet line.

Double Random Length

Double Random Length pipes are considerably longer than SRL pipes, with an average minimum length of 35 feet. DRL pipes are typically double the length of SRL pipes. Because of their large size, DRL mostly includes tubing with diameters above 4 inches. 

However, double random length pipes are open to customization, as with most sizing. Order specifications can define their length.

For example, DRL ASTM A53 pipes are typically around 35 feet, while API 5L DRL pipes have an average length of 39 feet. When pipes exceed the 40 feet mark, they can be grouped in a sub-category of "longer than DRL pipes."

Characteristic Features Of Double Random Length (DRL) Pipes

  • Double Random Length covers random pipes above 4 inches in diameter.
  • While custom lengths are available for longer pipelines, double random pipe lengths typically refer to lengths between 36 feet to 39 feet. 
  • A Double Random Length pipe is generally expected to be twice the length of a Single Random Length pipe, with a minimum average length of 35 feet.
  • The pipe specification influences the DRL length of each tubing batch.
  • Due to production variables, 5% of all Double Random Length pipes include a length of 19 feet to 35 feet.
  • The minimum length for DRL pipes is 19 feet.

Cut Length

Cut length pipes are tubing customized for specific applications. The main purpose of these customizations is to reduce the need for welding at project sites.

They are made in lengths within +/- ⅛ inch and are available in sizes up to 80 feet. Since Cut lengths come in longer sizes, there is no need to fuse pipes at multiple points; it helps save on installation costs.