Rule of thumb · applies
DN is not NPS, PN is not Class – why translating between the standards worlds is only an approximation
DN is not NPS, PN is not Class – only the dimensional drawing says what fits.
A rule of thumb replaces neither the design calculation nor the applicable standard.
- Category
- robust rule
- Standards referenced
- EN ISO 6708 (DN), EN 1333 (PN), EN 10220, EN 1092-1 – ASME B36.10M (NPS), ASME B16.5 (Class) – dimensions from catalogues
Rule of thumb
The nominal sizes of both worlds often name the same pipe, the pressure ratings never the same flange.
Where does the rule come from?
Whoever translates between the EN and ASME worlds reaches for tables “DN 50 = NPS 2, PN 16 ≈ Class 150”. The first equation holds as a name, the second is a legend: Class 150 and PN 16 lie in a similar range at room temperature but have different pressure-temperature curves, outside diameters, bolt circles and numbers of bolts. From this legend “PN 20” arose – a trade designation no standard knows. The rule was born in the warehouse: a flange ordered as “PN 20” is a Class 150 flange, and it fits no PN 16 mating flange.
What is technically correct?
DN (EN ISO 6708) and NPS (ASME B36.10M) are index numbers, not dimensions; up to NPS 12 the number names neither outside nor inside, from NPS 14 the outside diameter in inches. The outside diameters of both series coincide in most sizes (60.3 / 88.9 / 114.3 mm) – not at NPS 3/4, 1, 1 1/4, 2 1/2, 5 and 12 (26.7 / 33.4 / 42.2 / 73.0 / 141.3 / 323.8 against 26.9 / 33.7 / 42.4 / 76.1 / 139.7 / 323.9 mm of series 1; EN 10220 lists 73.0 and 141.3 only as a further series). PN (EN 1333) and Class (B16.5) are pressure rating series with their own pressure-temperature tables per material group; no PN is a Class. What fits together is told by the dimension table of the specific size – the dimensional drawing, not the translation table.
Where does the rule end?
- Nominal sizes: as names DN 50 and NPS 2 are interchangeable; as dimensions only where the outside diameter is the same. Wall thicknesses (schedule against millimetres) almost never are.
- Pressure ratings: in individual nominal sizes bolt circle and number of holes of Class 150 and PN 16 happen to coincide (DN 50) – boltable does not mean matching: raised face, bolts and curves differ.
- EN 1759-1 is the way to order Class flanges in the EN world – with the dimensions of B16.5, not with PN dimensions.
- Threads: NPT is not R – own rule “Two turns of thread do not seal”.
What does the standard say?
EN ISO 6708 defines DN as an index number without unit, EN 1333 PN as the index of the pressure rating; ASME B36.10M defines NPS and schedule, ASME B16.5 the Class series 150 to 2500 with pressure-temperature tables per material group. None of the standards names a counterpart in the other world; the mapping is trade practice. EN 1759-1 adopts the B16.5 dimensions for Class flanges to a European standard.
DIN/EN versus ASME/ASTM
That is the rule itself: the EN world counts in DN and PN with metric dimensions, the ASME world in NPS and Class with inches. The outside diameters of the pipes are historically the same (the inch series was adopted in millimetres), the flange dimensions are not – ASME flanges at the same nominal size are mostly larger in outside diameter and have fewer, thicker bolts. The sister rules “Class 150 is not 150 psi”, “PN 16 is not 16 bar”, “DN is not a diameter” and “Schedule is not a wall thickness” say per world what the number is not.
Worked example
Enquiry “flange PN 20 DN 100”: meant is Class 150 NPS 4 to B16.5 – outside diameter 228.6 mm, bolt circle 190.5 mm, 8 bolts 5/8 inch. The PN 16 flange DN 100 to EN 1092-1 has 220 mm, bolt circle 180 mm, 8 bolts M16. Both are called “DN 100”, neither fits the other. We ask which counterpart sits in the line – and supply the flange to the world of the mating flange.
Zickwolff reference
No company lore for this rule.
Checked today
Verdict: applies (class A). The DN/NPS mapping is an identity of names, PN/Class a trade legend; neither is reliable without a dimension table.
Further reading
- Mixing standards worlds: what fits, what only appears to fit, what does not fit – fits, appears to fit, does not fit
- Standards translator: DN to NPS, PN to Class, identify threads, material counterpart – tool: DN to NPS, PN to Class with warning
- EN 1092-1 vs ASME B16.5: flange standards compared – EN 1092-1 versus ASME B16.5 in dimensions
- ASME B16.5 flange dimensions: Class 150 to Class 2500 (ANSI) – ASME flange dimensions per Class
- Class 150 is not 150 psi – what the ASME pressure class really means – the sister rule on Class
- DN is not a diameter – why DN 50 never measures 50 mm – the sister rule on DN
Frequently asked questions
Is DN 50 the same as NPS 2?
As a name yes, both designate the pipe with 60.3 mm outside diameter. Wall thickness, tolerances and material are not fixed by it.
Which Class corresponds to PN 16?
None. Class 150 is the neighbour in range, not the counterpart; the flanges are not dimensionally identical in most nominal sizes.
What is PN 20?
A trade designation for Class 150 flanges. No standard knows it; orders are placed to ASME B16.5 Class 150 or EN 1759-1 Class 150.
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