Size of pipe is defined by Nominal Pipe Size.  They are defined in dimensional standards such as ASME B36.10 and ASME B36.19.

ASME B36.10 : Welded and Seamless Wrought Steel Pipes : Size Range 1/8″ to 80″

ASME B36.19 : Stainless Steel Pipe : Size Range : 1/8″ to 30″

Normally Stocked pipes are :

1/2, 3/4, 1, 1 1/2, 2, 3, 4, 5, 6, 8, 10, 12, 14, 16, 18, 20 and 24.

Seldom used Pipe Sizes 11/4, 2 1/2 3 ½ and 5 inch. (unusual sizes are sometimes required for connecting to equipment, but piping is normally run in the next larger stock size after connection has been made).

1/8, 1/4, 3/8 and 1/2-inch pipe is usually restricted to instrument lines or to service and other lines which have to mate with equipment.

1/2-inch pipe is extensively used for steam tracing and for auxiliary piping at pumps, etc.

Small Bore Pipes less than 2″ are normally either plain (PE). Big bore Pipes 2″ and above are Bevelled End (BE). Small Bore threaded end pipes are supplied with one coupling per length (‘threaded and coupled’, or ‘T&C’). Rating of the coupling needs to be specified.
Other types of ends, such as grooved for special couplings, can be obtained to order.

NPS 14 and larger pipe has outside diameter equal to the nominal pipe size.

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