A defined set of standard pipe sizes used in the US. Pipes are specified with two non-dimensional numbers for diameter based on the Imperial units of inches, and a schedule describing wall thickness. From the NPS and schedule number of a pipe, the pipe outside diameter and wall thickness can be obtained from reference tables. These are based on ASME standards. For example, NPS 14 Sch 40 has an outside diameter of 14 inches and a wall thickness of 0.437 inches. The European equivalent to NPS is DN (nominal diameter, diamètre nominal, or Durchmesser nach Norm), in which sizes are measured in millimetres. The term nominal bore (nb) is also used and is equivalent to NPS.