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NPS, DN and outside diameter — how pipe sizes convert

Pipe is specified by a nominal size, not its real bore. NPS (Nominal Pipe Size, inch-based) and DN (Diameter Nominal, the ISO/metric designation) are two names for the same nominal size — they are a label, and the actual outside diameter is fixed for a given size while the wall thickness changes with schedule. Only NPS 14 and above have an OD equal to the nominal inch value; below that the OD is larger than the name (e.g. NPS 2 has a 60.3 mm OD, not 50.8 mm).

Conversion method & standards

  • NPS ↔ DN ↔ OD from the dimensional tables of ASME B36.10M (carbon/alloy) and ASME B36.19M (stainless), with the DN designation per ISO 6708.
  • Wall thickness by schedule (Sch 5S…160, STD/XS/XXS) is read from the same standards; the inside diameter is ID = OD − 2×wall and the bore area = π/4×ID².
  • Schedule vs. wall: schedule numbers approximate 1000×(P/S), so a higher schedule is a thicker wall for the same OD — the bore shrinks and the pressure rating rises.

Every outside diameter and wall thickness is taken directly from the governing dimensional standard, not interpolated or assumed.

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