Difference between Pipes and Tubes

Pipes and tubes are not the same - there is a difference

Pipes

Since the purpose with a pipe is the transport of fluids like water, oil and many other products, the most import pipe property is the capacity, or in reality, the inside diameter of the pipe. The nominal diameter of a pipe is therefore related to  the inside diameter.

If we take a look at ASME/ANSI B 36.10 Welded and Seamless Wrought Steel Pipe, the inside diameter of a 2'' pipe schedule 40 is 2.067". The inside diameter of a schedule 80 pipe is 1.939". Both inside diameters are close to 2". The outside diameters for both schedules are 2.375".

Since the outside diameter of a single nominal pipe size is kept constant, the inside diameter of a pipe will depend on the "schedule", or the thickness, of the pipe. The schedule and the actual thickness of a pipe will vary with size of pipe.

It is common to identify pipes in inches by using NPS or "Nominal Pipe Size". The metric equivalent is called DN or "diametre nominel". The metric designations conform to International Standards Organization (ISO) usage and apply to all plumbing, natural gas, heating oil, and miscellaneous piping used in buildings. The use of NPS does not conform to American Standard pipe designations where the term NPS means "National Pipe Thread Straight".

Nominal Bore (NB) may be specified under British standards classifications along with schedule (wall thickness).

The tolerances are looser to pipes compared with tubes and  they are often less expensive to produce.

Tubes

The nominal dimensions of tubes are based on the outside diameter. If we look at Copper Tubes - ASTM B88 the outside diameter of a 2" pipe is 2.125", relatively close to 2".

The inside diameter of a tube will depend on the thickness of the tube. The thickness is often specified as a gauge. If we look at Copper Tubes - ASTM B88 the wall thickness of 0.083"of a 2" pipe is gauge 14.

The tolerances are higher with tubes compared to pipes. Tubes are often more expensive to produce than pipes.

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