The word "steel pipe" refers to round hollow sections used for transmission and distribution pipelines and piping systems that convey fluids and gases - such oil & gas, propane, steam, acids and water. These are products that find wide application in the petrochemical, oil & gas, marine and process industries.

Pipe inside diameter The most important dimension for a steel pipe is the inside diameter (called "pipe ID") which is expressed by a nominal size ("NPS" or "DN" in Europe, i.e. nominal diameter). The NPS is a rough indication of the fluid conveyance capacity of the hollow section (the diameter is also called bore size); the actual external diameter does not always match the nominal size (a 2 inch pipe, for instance, has an internal flow capacity of approximately 2 inches, but has an actual external diameter of 2.375 inches). For a specific NPS, the outside diameter is constant wherease the wall thickness increases at incremental schedules (so, the thicker the pipe, the smaller its inside diameter at any specific NPS value). For pipes, the most important mechanical parameters are the pressure rating, the yield strenght and the ductabilty.


Seamless pipes  Welded pipes
Seamless pipes               ERW pipes


The word "steel tube" is generally associated to round / square / rectangular hollow sections used for the construction of pressure equipment (such as boilers, heaters and superheaters), the deployment of instrumentation systems and the realization of infrastructures (mechanical and structural tubes). For all these applications, the exact size of the outer diameter of the hollow section is critical (whereas the inner bore is more important for steel pipes) and the mechanical properties (yield, tensile strenght and elongation) are key. Steel tubes for pressure equipment are available in relatively short outside diameters ranges (up to 5 inches), whereas tubulars for mechanical applications are available in a wider dimensional range. 

Tube outside diameterThe most important dimension for a steel tube is the outside diameter ("tube OD"), which is expressed in inches or millimeters and the wall thickness; the wall thickness (WT) of the steel tube can be expressed either in BWGSWG or inches/millimeters (these are all alternative indications of the wall thickness of steel tubes). The difference between the outside diameter and the wall thickness (multiplied by two) defines the inside diameter of the tube. 

In terms of pricing, steel tubes are generally more expensive than steel pipe due to their stricter manufacturing tolerances and mills productiviity (tons produced by hour). The most important physical properties of steel tubes are the hardness, the tensile strenght and highly precise dimensions.



To summarize the difference between pipe and tube and the pipe meaning vs. tube meaning:





Is related to the internal size, which expresses the fluid conveyance capacity. Diameter is expressed in NPS (nominal pipe size) or DN (nominal diameter). NPS doesn't match with the true internal diameter in inches, the index is a loose and approximate indication of the actual measure.

Refers to the outside diameter, i.e. the OD (outside diameter). The measure represents the actual, or true, outer size of the tube. The actual physical diameter match exactly the OD size.  


Thickness of pipes is expressed in schedule (most common are Sch 40, Sch STD., Sch. XS/XH, Sch. XXS - but other schedules are possible, such as 5, 10, 20, 30, 60, 100, 120, 140, 160). The actual thickness of a given schedule varies by NPS.

Wall thickness (mm or inch or gauges). Gauge measures (BWG or SWG) are used for thinner wall thicknesses, whereas larger measures are expressed with fractional or inch indexes.

Section Round only Round, rectangular, squared

Production range

Extensive (up to 80 inch and above)

Rather narrow both in terms of OD and thickness

Tolerances (stainghtness, dimensions, roundness, etc)

Set but not too restrictive.

Very strict. Production process is generally lenghty and involves many quality checks around key dimensional parameters such as straightness, roundness, wall thickness, surface, etc. 

Mills productivity

High yield (tons / hour) and large lots. Pipes are produced on large scale according to highly productive processes.

Low yield and small lots (due to relatively small dimensions and cumbersome inspection and quality control procedures throughout the manufacturing process)

Delivery time

Can be short

Generally longer

Market price

Relatively lower price than steel tubes

Higher due to productivity, strict tolerances, inspection requirements, ...


Most common is carbon (API 5L, ASTM A106 / A53) and low alloy steels (ASTM A335) 

From alloy steel upwards (ASTM A209, A213)

Joint connections

Laborious to ensure duration 

Quick (threaded ends, couplings, etc).

 Pipes and tubes specification.pdf


 Difference between pipes and tubes.pdf

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