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Every international valve order eventually hits the same confusion: the European engineer writes "DN100" while the North American spec says "4 inch", and someone assumes they are the same physical measurement. They are equivalent – not identical. Misunderstanding this is one of the most common causes of mis-ordered valves, flange mismatches and on-site leaks. Here is what DN and NPS actually mean, a usable conversion chart, and the traps to avoid.
Crucially, neither DN nor NPS equals the actual inside or outside diameter. They are compatibility labels – the "clothing size" that lets pipes, valves and flanges of the same tag fit together.
For a given NPS, the outside diameter stays constant while the inside diameter changes with wall thickness (schedule). An NPS 2 pipe is about 60.3 mm outside, whether it is Sch 10 or Sch 80 – but the bore shrinks as the wall thickens. The same logic applies to DN: a DN100 valve is a DN100 valve, but its real bore depends on the pressure class. So sizing a valve "by the number" without checking the schedule and standard is how you end up with the wrong bore or a flange that will not line up.
For large sizes from NPS 8" upward, the "rule of 25" is a close shortcut: DN ≈ NPS × 25 (10" → DN 250, 24" → DN 600). Below 4" the relationship is not linear, so use the chart. Note also that for NPS 14" and larger the NPS number does equal the actual OD in inches; below that, the OD is larger than the NPS number.
A DN50 valve and an NPS 2" valve are treated as the same size – but the actual ODs differ by about half a millimetre (60.5 vs 60.3 mm), and more importantly the flange drilling is governed by different standards. A DN50 valve built to EN 1092 will not mate with an ASME B16.5 Class 150 flange even though "2 inch" is the paired size. The size tag is only the starting point; the flange standard and pressure rating decide whether the bolts actually line up.
Japanese JIS "A" sizing uses the same numbers as DN – 15A = DN15 = 1/2", 50A = DN50 = 2" – common on Japanese-spec equipment. Russian GOST pipes use reduced outer diameters (a GOST DN100 is about 108 mm vs 114.3 mm ASME), so CIS projects need transition flanges. Taiwanese trade slang uses "fen" (points): 4 fen = 1/2" = DN15, 6 fen = 3/4" = DN20. When a supplier mixes these, confirm the actual flange drilling, not just the size word.
No – they are equivalent, not identical. DN50 and NPS 2 are paired as the same nominal size, but the actual outside diameter of an NPS 2 pipe is about 60.3 mm, while a DN50 steel pipe is about 60.5 mm. The pairing works for selection and flange matching, but the numbers are nominal labels, not measured millimetres or inches. Always confirm the flange drilling and face-to-face against the relevant standard.
No. DN (Diameter Nominal, ISO 6708) is a dimensionless reference number for a size series – it approximates the bore in millimetres but is not the actual ID or OD. For a given DN, the actual inside diameter changes with wall thickness (schedule): a DN100 Sch 40 pipe has a smaller bore than a DN100 Sch 10 pipe, even though both are 'DN100'.
Use a conversion chart (DN 15 ≈ 1/2", DN 50 ≈ 2", DN 100 ≈ 4", DN 150 ≈ 6", DN 200 ≈ 8", DN 300 ≈ 12"). For large sizes from NPS 8" up, the 'rule of 25' is a close shortcut: DN ≈ NPS × 25 (e.g. 10" → DN 250, 24" → DN 600). Below NPS 4" the relationship is not linear, so use the chart, not the formula.
Three things: the flange standard (EN 1092 / DIN vs ASME B16.5), the pressure rating (PN vs Class – they are not interchangeable), and the bore type (full bore vs reduced bore). A 'DN100' valve will not bolt to an ANSI Class 150 flange even though 4" is the paired size, and a reduced-bore valve changes the effective flow area. Match all three before ordering.