ASME vs EN Flanges: Key Differences

ASME B16.5 and EN 1092-1 are the two dominant pipe flange standards worldwide. They serve the same function but define completely different dimensions, bolt patterns, and pressure rating methodologies. Flanges built to one standard cannot be bolted to flanges built to the other. This fundamental incompatibility is the single most important thing to know when working on international projects.

Feature	ASME B16.5	EN 1092-1
Size designation	NPS (nominal pipe size)	DN (nominal diameter)
Pressure designation	Class (150, 300, 600…)	PN (10, 16, 25, 40…)
Size range	NPS 1/2-24	DN 10-4000
Large flanges	B16.47 (NPS 26-60)	Same standard (to DN 4000)
RF height (low class)	1.6 mm	2.0 mm
Bolt holes	Imperial (inches)	Metric (mm)
Bolt sizing	Imperial (e.g., 3/4”, 7/8”)	Metric (e.g., M20, M24)
Dimension master	Inch (metric is conversion)	Metric (mm)
P/T rating basis	Material-specific tables	Material group tables
Geographic use	Americas, Asia (oil & gas)	Europe, Middle East, Africa

Dimensional Comparison: DN 150 / NPS 6

To illustrate how different these standards are, here is a comparison of equivalent-size flanges:

Parameter	ASME B16.5 Class 150	EN 1092-1 PN 16
Flange OD	279.4 mm (11.00 in)	285 mm
Bolt circle	241.3 mm (9.50 in)	240 mm
Number of bolts	8	8
Bolt size	3/4 in (19.05 mm)	M20 (20 mm)
Bolt hole diameter	22.4 mm	22 mm
Thickness (WN)	22.4 mm	22 mm

These particular sizes are tantalizingly close—but not close enough. The bolt hole diameters and positions differ by fractions of a millimeter, and bolt threading (UNC vs. metric) is incompatible. Attempting to bolt them together risks misalignment, uneven gasket compression, and leaks.

Common Mistake

Some DN 150 PN 16 and NPS 6 Class 150 flanges “almost” fit together. They may even appear to bolt up on the bench. But the bolt holes will not align properly under operating conditions, and the joint will not pass leak testing. Never mix standards.

Pressure Rating Differences

The two systems use different methods to rate pressure:

• ASME B16.5: Pressure-temperature tables for 35 material groups. Ratings decrease with increasing temperature. See the full rating guide.
• EN 1092-1: PN number approximates the bar rating at ambient temperature. Pressure-temperature derating follows EN 13445 or EN 12516.

Service	ASME	EN Equivalent
Low-pressure utility	Class 150	PN 16 or PN 20
Medium-pressure process	Class 300	PN 40 or PN 50
High-pressure process	Class 600	PN 100
Very high pressure	Class 1500	PN 160 or PN 250

Pro Tip

On international projects, the piping specification should clearly state ASME or EN throughout. If a project uses ASME flanges, all gaskets, bolts, and mating components must also be ASME. The same applies to EN systems. Mixing creates problems at every level—procurement, fabrication, and construction.

Material Equivalence

ASME Material	EN Material	Type
A105	P250GH (EN 10222-2)	Carbon steel
A350 LF2	P285NH (EN 10222-4)	Low-temp carbon steel
A182 F304	1.4301 (EN 10222-5)	Austenitic stainless
A182 F316	1.4401 (EN 10222-5)	Austenitic stainless
A182 F316L	1.4404 (EN 10222-5)	Austenitic stainless

These materials are similar but not identical. Chemical composition limits and mechanical property requirements differ between ASTM and EN specifications. Dual-certified materials (meeting both ASTM and EN requirements) are available from major mills.

Read the full guide to flanges