There are two common pipe size standards: the American (ANSI/ASME/API) standard, which is in imperial units, and the European (DIN) system which uses metric units. In the American system, the pipe diameter is known as "Nominal Pipe Size" (NPS) or "Nominal Bore" (NB). In the European system, it is known as the "Nominal Diameter" (DN). The most common standard diameters are as follow: 0.5 inch (15 mm), 0.75 inch (20 mm), 1 inch (25 mm), 1.5 inch (40 mm), 2 inch (50 mm), 3 inch (80 mm), 4 inch (100 mm), 6 inch (150 mm), 8 inch (200 mm), 10 inch (250 mm), 12 inch (300 mm), 14 inch (350 mm), 16 inch (400 mm), 18 inch (450 mm), 20 inch (500 mm), 22 inch (550 mm) and 24 inch (600 mm) - further sizes up to 36 inch are also available. It should be noted that some intermediate standard pipe sizes are available commercially, such as 5 inch (125 mm), however these are less common.

For pipe sizes with a NB of 14 inch (DN 350 mm) and above, the nominal bore is the same as the pipe Outside Diameter (OD). Confusingly, for pipe sizes with a NB of 12 inch (DN 300 mm) and below, the nominal bore and outside diameter are different. For example, a pipe with a 12 inch NB (DN 300 mm) has an OD of 12.75 inches, or 324 mm.

Pipe Schedule
The pipe schedule sets the pipe wall thickness. Obviously increasing the wall thickness of the pipe
increases the mechanical strength of the pipe, allowing it to handle higher design pressures. The following pipe schedules are available (in order of increasing wall thickness): 5S, 10S, 10, 20, 30, 40S, STD, 40, XS (Extra Strong), 60, 80, 100, 120, 140, XXS (Double Extra Strong) and 160. So for pipe of 12 inch NB (DN 300 mm) with a pipe schedule 5S, has a wall thickness of 0.156 inches (4 mm) . As the schedule is increased, so does the wall thickness. At a pipe schedule of 160, a 12 inch NB pipe has a wall thickness of 1.312 inches (33.3 mm). It should be noted that not all pipe schedules are available for all pipe sizes.

Calculation of Pipe Internal Diameter (ID)

For process engineers, the most important information is the pipe Internal Diameter (ID), as this is used in line sizing calculations. As was discussed above, for a given Nominal Bore or Diameter, the pipe OD remains constant. Thus as the pipe schedule changes, the internal diameter of the pipe changes. A spreadsheet for calculating pressure drops (in liquids) is given here.
The ID can be easily calculated, as long as the pipe NB / DN and schedule are known. The pipe ID is given by the pipe NB minus double the pipe wall thickness (which can be obtained from the pipe schedule).For example, for a 12 NB (DN 300 mm) pipe, schedule 40, the OD and wall thickness are respectively 12.75 inches (324 mm) and 0.406 inches (10.4 mm). Thus:
Pipe ID = 12.75 inches - 2 x 0.406 inches = 11.94 inches, or Pipe ID = 324 mm - 2 x 10.4 mm = 303.2 mm It is worth bearing in mind that wall thicknesses come within a specified tolerance, depending on the engineering standard used - a typical wall thickness tolerence is 12.5%. This means that the actual internal pipe diameter may vary slightly from that quoted above.