DN (Diamètre Nominal / Nominal Diameter) is a dimensionless reference number used internationally to describe pipe and fitting size — it corresponds closely to, but isn't identical to, the pipe's actual bore in millimetres. A DN{SIZE} flange is the standard size used on {SIZE}mm (nominally {INCH_EQUIVALENT}") pipework, and this page covers its dimensions across the three flange standards most commonly encountered:
PN (pressure number/nominal) — the European/ISO system, where the flange is rated to a nominal pressure (e.g. PN16) at a reference temperature, with the actual permissible pressure falling as temperature rises.
ANSI (Class) — the American system, where the flange is rated by class (e.g. Class 150, Class 300) rather than a bar pressure figure directly.
BS10 — an older British flange standard, still specified on some legacy plant and equipment, using its own table/class designation (e.g. Table D, Table E).
The chart below gives the flange outside diameter, bolt hole PCD (pitch circle diameter), bolt hole size, and flange thickness for DN{SIZE} across these standards. Always match the flange standard, pressure rating, and bolt pattern exactly to the mating flange — a PN16 flange will not safely bolt up to an ANSI Class 300 flange of the same nominal bore without a proper reducing or adaptor arrangement, even where the bolt circle happens to look close.
Dimensional tables for PN, ANSI and BS10 DN400mm (16 Inch) flanges.
(Only link to the sizes you actually have pages for — please check this list against your own site, I've built it from what I could confirm plus the standard DN series, so it may include sizes you don't have a page for yet, or miss ones you do.)
DN is a nominal reference size, not a precise measurement of any single dimension of the pipe or fitting. It loosely tracks the bore in millimetres for smaller sizes, but shouldn't be read as an exact bore diameter — always check the actual dimensional chart for the pipe or fitting standard in question.
Can I bolt a PN-rated flange directly to an ANSI-rated flange?
Not as a rule — even where the nominal size matches, the bolt hole PCD, number of bolts, and bolt hole size can differ between PN and ANSI flanges of the "same" DN size. Always check the actual dimensions on both flanges before assuming they'll mate, and use a certified adaptor or reducing flange if they don't.
What's the difference between PN and Class ratings?
PN (pressure nominal) gives an approximate pressure rating in bar at a reference temperature; ANSI Class (150, 300, 600, etc.) is a separate rating system that doesn't correspond to a bar value directly — the actual allowable pressure for a given class depends on the flange material and operating temperature. Never assume a numerical equivalence between the two systems without checking the relevant pressure-temperature rating tables.
Is BS10 still used on new equipment?
BS10 has largely been superseded by PN (EN 1092) flanges in the UK and Europe, but it's still found on older plant, and replacement or matching flanges are sometimes still needed to BS10 dimensions for compatibility with existing pipework.
Affiliate Partners
Help support us by using our affiliate links below.
