Class 150 Quick Reference

Dimensions shown are ASME B16.5 Class 150 reference dimensions in inches. Flange type affects bore, hub, length and other geometry. Pressure class is not a direct psi rating. The complete NPS 1/2 to 24 table is in the Class 150 section below.

ASME B16.5-2025 covers pressure-temperature ratings, materials, dimensions, tolerances, marking, testing and the designation of openings for pipe flanges and flanged fittings, NPS 1/2 through NPS 24. It gives requirements in both metric and U.S. customary units, with bolt and bolt-hole diameters expressed in inch units. It also includes requirements and recommendations on flange bolting, gaskets and joints.

B16.5 is limited to flanges and flanged fittings made from cast or forged materials, plus blind flanges and certain reducing flanges made from cast, forged or plate materials.

The table covers every NPS from 1/2 to 24 in. Raised-face height is separate from the flange body.

Class 300 dimensions are not obtained by multiplying Class 150 dimensions. Compare NPS 4: Class 150 has a 9.00 in. OD, 7.50 in. bolt circle and 0.75 in. holes, while Class 300 has a 10.00 in. OD, 7.88 in. bolt circle and 0.88 in. holes.

Class 150 vs. Class 300 for NPS 4

Dimensions become class-specific from Class 400 up. Thickness generally increases, bolting changes, and the OD and bolt pattern may change. Class 2500 stops at NPS 12 under B16.5. This page does not republish those tables, because missing values should come from the standard and not from guesses.

For the higher classes, ASME convention is to list thickness without the 1/4 in. raised face, unlike Classes 150 and 300, which include 1/16 in. Always read the source table legend.

Class 150 does not mean 150 psi. The class is a rating designation, and the allowable pressure depends on the material group and the temperature. As an illustration, a Class 150 flange in A105 carbon steel (Material Group 1.1) is rated 285 psig from -20°F to 100°F, and the rating falls as temperature rises.

NPS identifies the connected piping size and not the physical flange diameter. For NPS 4 Class 150, the flange OD is 9.00 in. The nominal size of 4 is a designation, and the pipe itself measures 4.500 in. across. Complete NPS-to-OD data is in the Pipe Outside Diameter Chart.

For NPS 4 steel pipe the OD is 4.500 in. for every common schedule. Wall thickness changes the ID. The weld-neck bore must match the pipe bore, while the flange OD and bolt pattern stay the same.

The Texas Flange table lists a standard weld-neck bore of 4.03 in. for NPS 4, which corresponds to standard-wall pipe. Wall thickness and ID are in the Pipe Wall Thickness Chart and the Pipe Inside Diameter Chart.

Flange OD and bolt pattern are determined mainly by NPS, flange class and the applicable standard, not by pipe schedule. A weld-neck flange is different: its bore is machined to suit the pipe wall, so Schedule 40 and Schedule 80 pipe of the same NPS and class share the same bolt-pattern envelope but need different weld-neck bore details. Do not describe schedule as irrelevant to flange selection. The Pipe Schedule Chart explains the schedule system.

For NPS 4 Class 150, BCD = 7.50 in., so R = 3.75 in. This is useful for layout and inspection, but it does not replace the standard.

Number of holes and hole diameter are fixed by NPS and class. NPS 4 Class 150 has 8 holes of 0.75 in. on a 7.50 in. bolt circle. The Texas Flange table notes that the hole is 1/8 in. larger than the bolt, so the bolt diameters below are derived from the hole size. Flange holes are standard-specific, not ordinary clearance holes. For general references see the Bolt Hole Size Chart, the Hex Bolt Size Chart, the Nut Size Chart and the Washer Size Chart.

A weld-neck flange has a long tapered hub that is butt-welded to the pipe. Its important dimensions are the bore, hub diameter, overall length, flange OD and thickness. Match the bore to the pipe wall and schedule. Because hub and length dimensions are type-specific, read them from the weld-neck column of the standard or the manufacturer table.

The pipe passes into the slip-on flange bore, and the flange is fillet-welded. Its bore and hub geometry differ from a weld-neck flange, so the same NPS and class do not mean every dimension equals a weld-neck flange.

Socket-Weld Flange

The pipe is inserted into a socket, so the socket and bore dimensions govern.

Threaded Flange

A threaded bore connects the pipe with no butt weld. Check the thread requirements. Neither type is a weld-neck flange without the neck.

A lap-joint flange is used with a stub end and can rotate around it, which helps with bolt-hole alignment. Its bore and hub geometry are different from other types. Stub-end dimensions are outside this page.

A blind flange has no through bore and closes a flanged opening. Its OD and bolt pattern match the mating flanges for the same NPS and class, as shown below. Its thickness follows the separate blind-flange table in the standard and should not be assumed equal to the weld-neck or slip-on thickness, so thickness values are not republished here. Blind flanges also get substantially heavier at larger sizes.

Raised Face (RF)

The gasket contact area is raised above the main flange face. In ASME B16.5 it is about 1/16 in. (1.6 mm) for Classes 150 and 300, and about 1/4 in. (6.4 mm) for Classes 400 through 2500.

Flat Face (FF)

The gasket surface lies in the same plane as the flange face.

RF and FF flanges are not interchangeable in every joint. Do not confuse raised-face height with flange thickness: for Classes 150 and 300 the tabulated minimum thickness includes the 1/16 in. raised face.

A ring-type joint (RTJ) flange has a machined ring groove and uses a metallic ring gasket in higher-pressure or specified services. Groove and ring dimensions are standard-specific, and ASME B16.20 covers metal ring-joint gaskets. A complete RTJ groove table is not reproduced here.

Gasket selection depends on NPS, class, facing, gasket type, material and service. ASME B16.20-2023 covers metallic gaskets (ring-joint, spiral-wound and metal-jacketed) and ASME B16.21-2021 covers nonmetallic flat gaskets, both dimensionally suitable for the referenced flange standards. This page gives context and not a gasket size chart. After assembly, tightening follows the project procedure; the Bolt Torque Chart is a general fastener reference and not a flange procedure.

ASME B16.47-2025 covers large-diameter steel flanges NPS 26 through 60 in Classes 75, 150, 300, 400, 600 and 900. B16.5 covers NPS 1/2 through 24, so do not extend a B16.5 chart beyond NPS 24.

NPS 26-60

B16.47 contains blind and weld-neck flanges for these sizes.

Series A vs. Series B

B16.47 has two flange systems, Series A and Series B, with different OD, bolt circle and bolt count at every size. They do not bolt together, so do not combine them in one table. As one Series A example, an NPS 26 Class 150 flange has a 34.25 in. OD, a 31.75 in. bolt circle, 24 bolts and 1-1/4 in. studs.

For the NPS 4 Class 150 OD: 9.00 × 25.4 = 228.6 mm. B16.5 gives both U.S. customary and metric requirements, with bolt and hole diameters in inches. Converting an ASME flange to millimeters does not make it equivalent to an EN or DIN PN flange.

1. Identify NPS.
2. Identify the applicable standard (B16.5 up to NPS 24, B16.47 above).
3. Identify the pressure class.
4. Identify the flange type.
5. Identify the facing.
6. Identify the material.
7. Determine pipe OD and schedule where relevant.
8. Verify the flange bore and hub.
9. Verify the bolt pattern.
10. Select compatible gasket and bolting.
11. Check the pressure-temperature rating.
12. Verify the project specification and code.

The lookup covers only the Class 150 and 300 reference tables on this page. Confirm against ASME B16.5 and the project specification.

Limitations

Pipe References

Bolting and Hardware