So, what are the right ways to express the dimensions of a steel pipe?

For your better understanding, we will explain from the following aspects:

The 3 elements of pipe dimension
Dimension Standards of carbon and stainless steel pipe (ASME B36.10M & B36.19M)
Pipe Size Schedule (Schedule 40 & 80 steel pipe means)
Means of Nominal Pipe Size (NPS) and Nominal Diameter (DN)
Steel Pipe Dimension Chart (Size chart)
Pipe Weight Class Schedule (WGT)

And finally, we will tells you how to describe steel pipe dimensions & sizes correctly.

Steel Pipe Dimension 3 elements

A complete 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 elements we could calculate the pipe weight, how much pressure the pipe could bear, and the cost per foot or per meter.

So that’s why we always need to know the correct pipe size.

Steel Pipe Dimensions Chart (Size Chart)

Pipe Schedule Chart unit in mm as below, view here for Pipe Schedule Chart in inch.

Dimension standards for steel pipe

There are different standards to describe the steel pipe size, OD and wall thickness, which mainly are ASME B 36.10, ASME B 36.19.

Relevant standard specification ASME B 36.10M and B 36.19M

Both ASME B36.10 and B36.19 are the standard specifications for the dimensions of the steel pipe and accessories.

ASME B36.10M

The standard covers the standardization of steel pipe dimensions and sizes, including seamless or welded types, which are used in high or low temperatures and pressures.
The pipe is distinguished from a tube (Pipe vs Tube). In this case, the pipe is specifically used for pipeline systems and the transmission of fluids such as oil and gas, water, and slurry. The standard used is ASME B 36.10M.
In this standard, when the pipe’s Outer Diameter is smaller than 12.75 in (NPS 12, DN 300), the pipe’s actual diameter is larger than NPS (Nominal Pipe Size) or DN (Nominal Diameter).

As for steel tube dimensions, the actual outside diameter is the same as the pipe number for all sizes.

Why pipes with sizes smaller than NPS 12 (DN 300) have different OD

First, we need to acknowledge that all pipe sizes should be identified as nominal pipe size (NPS).

Secondly, for NPS 1/8 (DN 6) to NPS 12 (DN 300), these sizes are based on a standard outside diameter (O.D.). Initially, the O.D. was chosen to specify the pipe dimensions, but since pipes always have a wall thickness and internal diameter (I.D.), the difference between the O.D. and I.D. varies for small pipes. However, as the pipe dimensions increase, the O.D. and I.D. become approximately equal.

Since there is no direct relationship between the old standard thickness (O.D.) and Nominal Size, both of them are accepted according to the ASME B 36.10 M standard.

Steel Pipe Schedule

In ASME B36.10M, Pipe schedule is identified as Standard (STD), Ex-Standard (XS), or Double Extra Strong(XSS); Or with Schedule No. 5, 10, 20, 30, 40, 80, 120, 160.

ASME B36.19M

ASTM B36.19M is for the stainless steel pipe dimensions, including seamless and welded types (same as the previous standard.)

This dimension standard is mostly the same as ASTM B36.10M. Wherever the different part is:
a. For NPS 14 to NPS 22 (DN 350-550), schedule 10S;
b. NPS 12 of Schedule 40S
c. NPS 10 and 12 of schedule 80S.

The above-mentioned pipe thickness is different with B36.10M, so suffix “S” is used here.

Ways to express pipe dimensions

1. For Pipe wall thickness: Use Steel Pipe Schedule, like schedule 40 steel pipe, schedule 80 pipe.
2. For pipe Diameters: Nominal Pipe Size (NPS) and DN (Nominal Diameter)
3. Pipe weight Class (WGT), LB/FT (Pounds per foot), KG/M (Kg per meter)

What is Steel Pipe Dimensions Schedule?

Steel pipe schedule is an indicating method represented by ASME B 36.10, and 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, where “80” is a pipe number from chart/table ASME B 36.10.

“Since the steel pipe’s main application is to transport the fluids under pressure, their internal diameter is their critical size. This critical size is taken as nominal bore (NB). Therefore, if steel pipe carries the fluids with pressure, it is particularly important that pipe shall have enough strength and 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.

What does SCH mean for the steel pipe dimensions

When describing the steel pipe parameter, we usually use the pipe schedule, which is a method that represents pipe wall thickness with number. Pipe schedule (sch. ) is not a wall thickness, but a wall thickness series. Different pipe schedules mean different wall thicknesses for the steel pipe in the same diameter. The most frequent indications of schedule are SCH 5, 5S, 10, 10S, 20, 20S, 30, 40, 40S, 60, 80, 80S, 100, 120, 140, 160. The larger the table number, the thicker the surface pipe wall, the higher the pressure resistance.

Schedule 40, 80 steel pipe dimension means

If you are new to the pipe industry, you may wonder why you always see schedule 40 or 80 steel pipes everywhere. What kind of material is used for these pipes?

As you have read above articles you know that Schedule 40 and 80 represent pipe wall thickness, but why are they always searched by buyers?

Here is the reason:

Schedule 40 and 80 steel pipes are commonly required in various industries due to the typical pressure they can withstand. Because these pipes are designed to handle higher pressures, they are often requested in large quantities to meet the demands of different industries.

The material standard for such thickness pipes has no limitations, you could ask for 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..

What is the Nominal Pipe Size (NPS)?

Nominal Pipe Size (NPS) is a North American set of standard sizes for pipes used for high or low pressures and temperatures. Pipe size is specified with two non-dimensional numbers: a nominal pipe size (NPS) based on inches, and a schedule (Sched. or Sch.).

What is DN (Nominal Diameter)

Nominal diameter, also known as outside diameter, is used to represent the general diameter of various pipes and pipeline accessories. Due to the thin wall of the pipe, the outside and inside diameters are almost the same. Therefore, the average value of both parameters is used as the nominal diameter of the pipe. DN (nominal diameter) ensures that pipes and pipe fittings with the same nominal diameter can be interconnected, providing interchangeability. Although the nominal diameter value is close to or equal to the inside diameter of the pipe, it does not represent the actual sense of the pipe diameter. The nominal size is denoted by a numerical symbol followed by the letter “DN” and the unit is indicated in millimeters after the symbol. For example, DN50 represents a pipe with a nominal diameter of 50 mm.

DN (mm) and NPS (inch) conversion

Nominal diameter DN (mm) and NPS (inch) conversion

1. Conversion DN ( mm ) that requires separate memory

2. Greater than or equal to DN100 diameter and NPS conversion

DN=25*NPS;
NPS=DN/25

3. Exact conversion

1 inch =25.4 mm

Pipe Weight Class Schedule

The WGT class (weight class) is an indication of the pipe wall thickness that was used in the past, but is still in use today. It consists of three grades: STD (standard), XS (extra strong), and XXS (double extra strong).

In the early days of pipe production, each size of pipe had only one specification, known as standard tube (STD). However, in order to handle high pressure fluids, thicker pipes known as XS were introduced. XXS pipes were then developed to handle even higher pressure fluids. As new materials processing technology emerged, there was a growing demand for more cost-effective thin-walled pipes. This eventually led to the introduction of the aforementioned pipe grades. The corresponding relationship between pipe schedules and weight classes can be found in the ASME B36.10 and ASME B36.19 specifications.

How to describe steel pipe dimensions and size correctly?

For example: a. Expressed as “pipe outside diameter × wall thickness”, such as Φ 88.9mm x 5.49mm (3 1/2” x 0.216” ).  114.3mm x 6.02mm (4 1/2” x 0.237”), length 6m (20ft) or 12m (40ft), Single Random Length (SRL 18-25ft), or Double Random Length (DRL 38-40ft).

b. Expressed as “NPS x Schedule”, NPS 3 inch x Sch 40, NPS 4 inch x Sch 40. Same size as above specification.

c. Expressed as “NPS x WGT Class”, NPS 3 inch x SCH STD, NPS 4 inch x SCH STD. Same size above.

d. “Pipe Outer Diameter x lb/ft.” is commonly used in North America and South America. For example, a pipe with an outer diameter of 3 1/2” would be described as having a weight of 16.8 lb/ft. The unit “lb/ft” stands for pounds per foot.

Then how to calculate the steel pipe weight per foot or per meter?

Through pipe weight and total quantity meters, we will get the whole weight for the pipe request. Another way to find out this information is through the steel pipe chart (Below the article is the steel pipe weight chart).

Steel pipe unit weight is another way to express the pipe size schedule, calculate the maximum pressure of the pipe, and do the budget for the whole line pipe project.

How to calculate steel pipe per foot or meter

Basically, there are two ways to calculate steel pipe weight per foot or per meter.
One is to calculate according to the steel pipe weight formula.
Another way is to search for the information from the steel pipe weight chart.

How to calculate the weight of steel pipe by formula

The steel pipe unit weight (kg/m or lb/ft) should be calculated according to the below formula.

kg/m is kilograms per meter

lb/ft is pounds per foot

P1= t(D-t)*C

Where
D is the specified outside diameter, expressed in millimeters (inches)
t is the specified wall thickness, expressed in millimeters (inches)
C is 0.02466 for calculations in SI units and 10.69 for calculations in USC units.

Please note: The nominal weight of a steel pipe is calculated by multiplying its length by its mass per unit length (per foot or per meter).

For example: The description of API 5L seamless steel line pipe, Outer Diameter is 6 5/8 inch (168.3 mm), Pipe Wall Thickness is SCH 40 (7.11 mm or 0.280 inch), length 12 meters.
Then the unit pipe weight kg/m, 7.11 x (168.3-7.11) x 0.02466 = 28.26 kg/m

Unit steel pipe weight lb per foot is 0.28 x (6-0.28) x 10.69 = 18.99 lb/ft

How to get pipe weight from steel pipe weight chart

Another method to find out pipe weight per foot is through steel pipe Dimensions and Weight chart, this chart is very useful in the pipeline activities.

This chart includes the information of pipe nominal diameters, wall thickness, Schedule (SCH) Class, as well as corresponding inf of pipe weight lb per foot or kg per meter.

Steel pipe wall thickness description

1). The wall thickness of API 5L line pipe has three representing method as following:

a. Pipe schedule number “Sch”

b. Steel pipe wall thickness in mm

c. Pipe weight kg/m or lb/ft

According to pipe weight, the pipe wall thickness is generally applied for three types:

Standard weight pipe in STD, the thickened pipe in XS, and extra thick pipe in XXS. (More pipe schedule is applicable here)

For pipes with DN≤250 mm, Sch40 is equivalent to STD,

For pipes with DN<200mm, Sch80 is equivalent to XS

2). Tolerances for pipe wall thickness

Pipe diameter

Tolerances for pipe diameter

Common unit conversation table applied in steel line pipe description

2. Steel pipe dimensions and weight chart

Steel pipe weight chart for download:

API 5L pipe length definition

In API 5L standard spec, the steel line pipe shall be delivered in accordance with the random length and approximate length in the order contract.
In API 5L specification 9.11.3.3 specified the tolerance for the the pipe length as belowed:
a. Unless other lengths agreed, (Length with less tolerances), random lengths shall be applied as the table 12.
b. Approximate lengths shall be delivered within a tolerance of +/-500 mm (+/-20 inch).
The common random length of API 5L steel line pipe is designated at 6 m, 8 m, 9 m, 10 m and 12 m. The length also can be customized according to specific requirements.

The tolerances for random length pipe shall be as below table (API SPEC 5L Table 12).

Is it easy to comply the pipe length tolerance

For the welded pipe the raw material is steel coils or steel plates, so it is easy to comply longer length with big diameter. But for seamless steel line pipe, raw material is steel round bar and manufactured by hot rolled processes, so the pipe length is limited by the raw material round bar and how big diameters line pipe required. If required longer length (more than 12 meters or 40 ft) with big diameters (specially OD more than 20” or 508 mm), it is difficult to comply with seamless.

How to handle seamless steel pipe with longer length and big diameters

Normally for Chinese seamless steel pipe manufacturer the maximum original diameter produced by hot rolled process is 16” 406 mm, few manufacturers are capable of produce maximum 18” seamless steel pipe, Octal could supply maximum 20” 508 mm original seamless steel line pipe by hot rolled process.

For the seamless steel line pipe outer diameters more than 508 mm, it shall be manufactured by hot expanding processes. (Octal supply seamless steel pipe maximum to 1200 mm).

Hot expanding process for seamless steel pipe

Hot expanding process is an after-treatment for the original hot rolled seamless steel pipe, it is the process to enlarge the pipe diameters by hot expanding. During the hot expanding process, the heater (normally the heat ring) makes the heat to the pipe body, by the help of the different Dia of hydraulic lever inside the pipe move through the pipe internal, the diameters will be expanded as required size.