Flange Dimensions — ASME B16.5

Six flange types across seven pressure classes, NPS 1/2 through NPS 24. Outside diameter, thickness, bolt circle, bolt count and bolt size for every combination.

Weld Neck FlangeWN · 7 classesHigh pressure, high temperature, cyclic loading, hydrogen and lethal service.Slip-On FlangeSO · 6 classesLow-pressure, non-cyclic utility and water service.Socket Weld FlangeSW · 5 classesSmall-bore high-pressure lines: instrument, hydraulic and steam trace piping.Lap Joint FlangeLJ · 7 classesSystems dismantled often, or alloy lines where flange cost matters.Threaded FlangeTHD · 7 classesLow-pressure utility piping and hazardous areas where welding is banned.Blind FlangeBL · 7 classesVessel and line closures, future tie-in points, pressure-test blanks.

Weld Neck Flange

Slip-On Flange

Socket Weld Flange

Lap Joint Flange

Threaded Flange

Blind Flange

The six ASME B16.5 flange types in section. Flange metal is hatched, the pipe is grey and welds are orange. In one size and class all six have the same outside diameter, bolt circle and bolting; they differ in how they are fixed to the pipe. Not to scale.

What the pressure class buys you

The same NPS 6 flange across all seven classes, so the jump in metal and bolting is visible at a glance. Ratings are for A105 carbon steel at 100 °F.

Dimensions are given in inches, with the millimetre equivalent beneath in grey.

NPS 6 flange dimensions by pressure class, ASME B16.5.
Class	Flange OD	Thickness	Bolt circle	Bolting	Rating @ 100 °F
Class 150	11.00279.40 mm	0.9423.88 mm	9.50241.30 mm	8 × 3/4 in	285 psig
Class 300	12.50317.50 mm	1.3835.05 mm	10.62269.75 mm	12 × 3/4 in	740 psig
Class 400	12.50317.50 mm	1.6241.15 mm	10.62269.75 mm	12 × 7/8 in	985 psig
Class 600	14.00355.60 mm	1.8847.75 mm	11.50292.10 mm	12 × 1 in	1480 psig
Class 900	15.00381.00 mm	2.1955.63 mm	12.50317.50 mm	12 × 1 1/8 in	2220 psig
Class 1500	15.50393.70 mm	3.2582.55 mm	12.50317.50 mm	12 × 1 3/8 in	3705 psig
Class 2500	19.00482.60 mm	4.25107.95 mm	14.50368.30 mm	8 × 2 in	6170 psig

Browse by size

Every pressure class published in one nominal size, shown on the page for that pipe size alongside its schedules and fittings — the quickest route if you already know the size and need to compare classes.

Larger than NPS 24

Flanges from NPS 26 to NPS 60 fall under ASME B16.47, in two series that in general will not bolt to each other. Large diameter flanges →

Guides and comparisons for flanges

Pressure-temperature deratingReading a P-T table and interpolating between temperatures.Choosing a flange face typeRaised face, flat face or ring-type joint, by service.Flange bolt torqueTarget torque from bolt stress, for every B16.5 bolt size.Weld neck vs slip-on flangeOne butt weld against two fillet welds.Class 150 vs Class 300Dimensions, bolting and ratings of the two commonest classes.Raised face vs ring-type jointGasket seating at low and high pressure class.

Common questions

What does the class number on a flange mean?

It is a dimensional and rating designation, not a pressure. A Class 300 flange is not rated for 300 psi — in A105 carbon steel it is rated 740 psig at 100 °F, and 570 psig at 600 °F. The number derives from the historic rating of the class in saturated steam service.

How do I know which flange type I need?

Work down from service severity. High pressure, cyclic loading or high temperature calls for a weld neck. Ordinary low-pressure utility service can take a slip-on. Small-bore high-pressure lines use socket weld. Frequent dismantling or expensive alloy favours lap joint. Non-weldable locations get threaded. Closures get a blind.

Where does B16.5 stop and B16.47 start?

ASME B16.5 covers NPS 1/2 through NPS 24. From NPS 26 up, flanges are covered by ASME B16.47, which has two series, A and B, that in general do not interchange.