NPS, NB and DN — what the size really means

Nominal Pipe Size (NPS) is the North American size designation in inches. NB (nominal bore) is the same idea used across India and the UK, and DN (diamètre nominal) is the metric equivalent in millimetres.

From NPS ⅛ to NPS 12 the number is not the actual outside diameter: a 4" pipe has an OD of 114.3 mm. From NPS 14 upward, the OD equals the NPS in inches (NPS 14 → 355.6 mm OD).

The OD stays fixed for a size while the inside diameter changes with the wall thickness. That is why fittings and flanges for one NPS fit every schedule of that size.

Quick conversions: NPS ½ = DN 15, ¾ = 20, 1 = 25, 1½ = 40, 2 = 50, 3 = 80, 4 = 100, 6 = 150, 8 = 200, 10 = 250, 12 = 300, and DN ≈ NPS × 25 above that.