Class 300 Flange Dimensions (ASME B16.5)

Class 300 flanges per ASME B16.5 are used for moderate-to-high pressure piping services. They are the second most common flange pressure class after Class 150, frequently specified for process piping in refineries, petrochemical plants, and gas processing facilities.

When Class 300 Is Used

Class 300 flanges are specified when the design pressure exceeds the Class 150 rating at the operating temperature, or when the project piping specification requires a higher safety margin. At ambient temperature, a Class 300 carbon steel (ASTM A105) flange is rated for approximately 51 bar (740 psi).

Specs Table: Class 300 WN Flange Dimensions (ASME B16.5)

NPS	DN	Flange OD (mm)	Thickness (mm)	Raised Face Dia. (mm)	Bolt Circle (mm)	No. of Bolts	Bolt Dia. (in)	Bolt Hole Dia. (mm)
1/2	15	95.3	14.2	34.9	66.5	4	1/2	15.7
3/4	20	117.5	15.7	42.9	82.6	4	5/8	19.1
1	25	124.0	17.5	50.8	88.9	4	5/8	19.1
1-1/4	32	133.4	19.1	63.5	98.6	4	5/8	19.1
1-1/2	40	155.4	20.6	73.2	114.3	4	3/4	22.4
2	50	165.1	22.4	92.1	127.0	8	5/8	19.1
2-1/2	65	190.5	25.4	104.6	149.4	8	3/4	22.4
3	80	210.0	28.4	127.0	168.1	8	3/4	22.4
4	100	254.0	31.8	157.2	200.2	8	3/4	22.4
5	125	279.4	35.1	185.7	235.0	8	3/4	22.4
6	150	317.5	36.6	215.9	269.7	12	3/4	22.4
8	200	381.0	41.1	269.7	330.2	12	7/8	25.4
10	250	444.5	47.8	323.9	387.4	16	1	28.4
12	300	520.7	50.8	381.0	450.9	16	1-1/8	31.8
14	350	584.2	53.8	412.8	514.4	20	1-1/8	31.8
16	400	647.7	57.2	469.9	571.5	20	1-1/4	35.1
18	450	711.2	60.5	533.4	628.7	24	1-1/4	35.1
20	500	774.7	63.5	584.2	685.8	24	1-1/4	35.1
24	600	914.4	69.9	692.2	812.8	24	1-1/2	41.1

Dimensions per ASME B16.5-2020 for welding neck flanges. Raised face height is 6.4 mm (1/4 in) for Class 300, included in the thickness value.

Class 300 vs Class 150: Key Dimensional Differences

NPS	Class 150 OD (mm)	Class 300 OD (mm)	Class 150 Thickness (mm)	Class 300 Thickness (mm)	Class 150 Bolts	Class 300 Bolts
2	152.4	165.1	19.1	22.4	4 x 5/8”	8 x 5/8”
4	228.6	254.0	23.9	31.8	8 x 5/8”	8 x 3/4”
6	279.4	317.5	25.4	36.6	8 x 3/4”	12 x 3/4”
8	342.9	381.0	28.4	41.1	8 x 3/4”	12 x 7/8”
12	482.6	520.7	31.8	50.8	12 x 7/8”	16 x 1-1/8”

Class 300 flanges have larger ODs, thicker bodies, and more/larger bolts than Class 150 for the same NPS. This accommodates the higher pressure rating while maintaining a gasket seating stress adequate for reliable gasket performance.

Important Notes

Raised face height: Class 300 flanges have a raised face height of 6.4 mm (1/4 in), compared to 1.6 mm (1/16 in) for Class 150. This affects stud bolt length calculations.

Material impact on rating: The 51 bar (740 psi) rating applies to Group 1.1 materials (ASTM A105, A216 WCB). Higher-grade materials (e.g., ASTM A182 F304, Group 2.1) have higher pressure ratings at the same class.

Common Mistake

Using Class 150 bolt lengths on Class 300 flanges. Class 300 flanges are significantly thicker and have a taller raised face than Class 150. Always recalculate stud bolt lengths for the specific pressure class to ensure adequate thread engagement.

For complete welding neck flange dimensions across all ASME B16.5 pressure classes, see the reference tables.

Read the full guide to pipe flanges