Tube and Pipe - Understanding the differences

The fundamental difference between tube and pipe, which can often have the same outside diameter (O.D.), is that tube is manufactured to tighter tolerances than pipe.

Tube Diameter and Pipe Diameter sizes

Tube sizes are the actual outside diameter. (O.D.)
Pipe sizes are the nominal bore (N.B.). The true outside diameter of a Pipe is larger than its nominal bore size. Example:
4” N.B Pipe has an actual outside diameter of 4½” O.D or 114.3mm.
The outside diameter will remain the same but the actual bore size will change with wall thickness.

Tube Wall Thickness

Tube wall thicknesses will often be stated by their actual metric measurement.
Tube wall thickness can also be referred to by gauge, this comes from the Standard Wire Gauge (SWG) that was once commonly used. In recent years the use of SWG sizes has fallen greatly in popularity, the tube wall thicknesses are now more commonly measured in millimetres as more popular size measurement.

Pipe Schedules (American standards)

As the main function of the pipes is to carry fluid under pressure their wall thickness is the critical dimension. Wall thickness is expressed in “schedules“, referred to as pipe schedules. The pipe schedule is abbreviated as SCH. For a given size and schedule the outside diameter and thickness of the pipe is fixed and defined in the applicable ASME standard. Other than the pipe schedule, pipe thickness can also be specified in inches or mm to the value corresponding to that specified in the ASME standard.

Pipe Standards

ASME/ANSI B 36.10 Welded and Seamless Wrought Steel Pipe, and
ASME/ANSI B36.19 Stainless Steel Pipe

Stainless Steel Pipe Sizes

ASME/ANSI B36.19 Stainless Steel Pipe

PIPE SIZE SCHEDULE 5 SCHEDULE 10 SCHEDULE 40 SCHEDULE 80
N.B (Inch) O.D (mm) W/T (mm) W/T (mm) W/T (mm) W/T (mm)
1/8 10.29 1.25 1.73 2.41
¼ 13.72 1.65 2.24 3.02
3/8 17.15 1.65 2.31 3.20
½ 21.34 1.65 2.11 2.77 3.73
¾ 26.67 1.65 2.11 2.87 3.91
1 33.4 1.65 2.77 3.38 4.55
42.16 1.65 2.77 3.56 4.85
48.26 1.65 2.77 3.68 5.08
2 60.33 1.65 2.77 3.91 5.54
73.03 2.11 3.05 5.16 7.01
3 88.9 2.11 3.05 5.49 7.62
101.6 2.11 3.05 5.74 8.08
4 114.3 2.11 3.05 6.02 8.56
5 141.3 2.77 3.4 6.55 9.52
6 168.28 2.77 3.4 7.11 10.97
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