Class 300 Threaded Flange Dimensions

ASME B16.5 dimensions and bolting for the Class 300 threaded flange, NPS 1/2 through NPS 24, with what the class is rated for and where this flange type is used in it.

Standard: ASME B16.5
Flange type: Threaded Flange
Pressure class: Class 300
Size range: NPS 1/2 – NPS 24
Raised face height: 0.06 in (1.5 mm)
Rating at 100 °F (A105): 740 psig (51.0 bar)

Where a Class 300 threaded flange is used

Class 300 is the usual step up when Class 150 runs out of pressure or temperature. It is common on hydrocarbon process lines, steam and hot oil, and it is often chosen over Class 150 for a hazardous fluid at modest pressure simply for the heavier joint.

A threaded flange screws onto the pipe and needs no welding, which suits small utility lines and places where hot work is not allowed. It is normally kept to small sizes and steady service.

The number 300 is a designation and not a pressure. A Class 300 flange in A105 carbon steel is rated 740 psig (51.0 bar) at 100 °F, 635 psig at 400 °F, 570 psig at 600 °F and 410 psig at 800 °F. In Type 304 stainless, Group 2.1, it starts at 720 psig and still holds 405 psig at 800 °F, so the material and the metal temperature have to be fixed before the class can be.

Left: threaded flange in section, showing what each column of the table measures. Welds are orange; the section is not to scale. Right: an NPS 6 Class 300 flange from the front, to scale. It is 12.50 in (317.5 mm) across, with 12 holes of 0.875 in for 3/4 in bolts on a 10.62 in (269.7 mm) bolt circle. The holes straddle the centrelines.

Dimensions in every size

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

ASME B16.5 Class 300 threaded flange dimensions.
Size	Flange OD	Thickness	Bolt circle	Bolts	Bolt dia.	Raised face OD	Length thru hub
NPS 1/2	3.7595.25 mm	0.5012.70 mm	2.6266.55 mm	4	1/2 in	1.3835.05 mm	0.8120.57 mm
NPS 3/4	4.62117.35 mm	0.5614.22 mm	3.2582.55 mm	4	5/8 in	1.6942.93 mm	0.9423.88 mm
NPS 1	4.88123.95 mm	0.6215.75 mm	3.5088.90 mm	4	5/8 in	2.0050.80 mm	1.0025.40 mm
NPS 1 1/4	5.25133.35 mm	0.6917.53 mm	3.8898.55 mm	4	5/8 in	2.5063.50 mm	1.0025.40 mm
NPS 1 1/2	6.12155.45 mm	0.7519.05 mm	4.50114.30 mm	4	3/4 in	2.8873.15 mm	1.1228.45 mm
NPS 2	6.50165.10 mm	0.8120.57 mm	5.00127.00 mm	8	5/8 in	3.6291.95 mm	1.2531.75 mm
NPS 2 1/2	7.50190.50 mm	0.9423.88 mm	5.88149.35 mm	8	3/4 in	4.12104.65 mm	1.4436.58 mm
NPS 3	8.25209.55 mm	1.0626.92 mm	6.62168.15 mm	8	3/4 in	5.00127.00 mm	1.6241.15 mm
NPS 3 1/2	9.00228.60 mm	1.1228.45 mm	7.25184.15 mm	8	3/4 in	5.50139.70 mm	1.6942.93 mm
NPS 4	10.00254.00 mm	1.1930.23 mm	7.88200.15 mm	8	3/4 in	6.19157.23 mm	1.8145.97 mm
NPS 5	11.00279.40 mm	1.3133.27 mm	9.25234.95 mm	8	3/4 in	7.31185.67 mm	1.9449.28 mm
NPS 6	12.50317.50 mm	1.3835.05 mm	10.62269.75 mm	12	3/4 in	8.50215.90 mm	2.0050.80 mm
NPS 8	15.00381.00 mm	1.5639.62 mm	13.00330.20 mm	12	7/8 in	10.62269.75 mm	2.3860.45 mm
NPS 10	17.50444.50 mm	1.8145.97 mm	15.25387.35 mm	16	1 in	12.75323.85 mm	2.5665.02 mm
NPS 12	20.50520.70 mm	1.9449.28 mm	17.75450.85 mm	16	1 1/8 in	15.00381.00 mm	2.8171.37 mm
NPS 14	23.00584.20 mm	2.0652.32 mm	20.25514.35 mm	20	1 1/8 in	16.25412.75 mm	2.9474.68 mm
NPS 16	25.50647.70 mm	2.1955.63 mm	22.50571.50 mm	20	1 1/4 in	18.50469.90 mm	3.1981.03 mm
NPS 18	28.00711.20 mm	2.3158.67 mm	24.75628.65 mm	24	1 1/4 in	21.00533.40 mm	3.4487.38 mm
NPS 20	30.50774.70 mm	2.4461.98 mm	27.00685.80 mm	24	1 1/4 in	23.00584.20 mm	3.6993.73 mm
NPS 24	36.00914.40 mm	2.6968.33 mm	32.00812.80 mm	24	1 1/2 in	27.25692.15 mm	4.12104.65 mm

Flange thickness excludes the raised face. Bolt diameter is nominal; stud bolts are normally ASTM A193 B7 with A194 2H nuts.

Across the 20 sizes published in Class 300, the flange grows from 3.75 in (95.2 mm) across and 0.50 in (12.7 mm) thick at NPS 1/2 to 36.00 in (914.4 mm) across and 2.69 in (68.3 mm) thick at NPS 24. The bolt count goes 4 from NPS 1/2, 8 from NPS 2, 12 from NPS 6, 16 from NPS 10, 20 from NPS 14 and 24 from NPS 18, always in multiples of four so that the holes can straddle the centrelines. The largest bolt in the class is 1 1/2 in, first needed at NPS 24. Against Class 150, an NPS 6 flange in Class 300 is 1.50 in larger in diameter and 1.5 times as thick.

Bolt holes, spacing and torque

Bolt holes in ASME B16.5 flanges are drilled 1/8 in larger than the nominal bolt and are set to straddle the vertical and horizontal centrelines, never to sit on them. Spacing is the distance between neighbouring holes measured round the bolt circle, which shows how much room there is for a wrench or a tensioner. The torque column is a starting figure for a lubricated stud, not a joint-specific value.

Bolting geometry for Class 300 flanges, derived from the ASME B16.5 bolt size and bolt circle.
Size	Bolting	Bolt hole dia.	Hole spacing	Torque for 50 ksi bolt stress
NPS 1/2	4 × 1/2 in	0.62515.88 mm	2.0652.27 mm	47 lb-ft
NPS 3/4	4 × 5/8 in	0.75019.05 mm	2.5564.83 mm	94 lb-ft
NPS 1	4 × 5/8 in	0.75019.05 mm	2.7569.82 mm	94 lb-ft
NPS 1 1/4	4 × 5/8 in	0.75019.05 mm	3.0577.40 mm	94 lb-ft
NPS 1 1/2	4 × 3/4 in	0.87522.22 mm	3.5389.77 mm	167 lb-ft
NPS 2	8 × 5/8 in	0.75019.05 mm	1.9649.87 mm	94 lb-ft
NPS 2 1/2	8 × 3/4 in	0.87522.22 mm	2.3158.65 mm	167 lb-ft
NPS 3	8 × 3/4 in	0.87522.22 mm	2.6066.03 mm	167 lb-ft
NPS 3 1/2	8 × 3/4 in	0.87522.22 mm	2.8572.32 mm	167 lb-ft
NPS 4	8 × 3/4 in	0.87522.22 mm	3.0978.60 mm	167 lb-ft
NPS 5	8 × 3/4 in	0.87522.22 mm	3.6392.26 mm	167 lb-ft
NPS 6	12 × 3/4 in	0.87522.22 mm	2.7870.62 mm	167 lb-ft
NPS 8	12 × 7/8 in	1.00025.40 mm	3.4086.45 mm	269 lb-ft
NPS 10	16 × 1 in	1.12528.57 mm	2.9976.06 mm	404 lb-ft
NPS 12	16 × 1 1/8 in	1.25031.75 mm	3.4988.52 mm	593 lb-ft
NPS 14	20 × 1 1/8 in	1.25031.75 mm	3.1880.79 mm	593 lb-ft
NPS 16	20 × 1 1/4 in	1.37534.92 mm	3.5389.77 mm	833 lb-ft
NPS 18	24 × 1 1/4 in	1.37534.92 mm	3.2482.29 mm	833 lb-ft
NPS 20	24 × 1 1/4 in	1.37534.92 mm	3.5389.77 mm	833 lb-ft
NPS 24	24 × 1 1/2 in	1.62541.27 mm	4.19106.40 mm	1,492 lb-ft

Torque is T = K·F·d with nut factor K = 0.16 and a bolt stress of 50,000 psi on the tensile stress area. The bolt torque guide gives the method and other nut factors.

Guides and comparisons for this page

Slip-on vs threaded flangeWelded or screwed attachment for low-pressure service.Socket weld vs threaded flangeThe two small-bore options compared.Class 150 vs Class 300Dimensions, bolting and ratings of the two commonest classes.Class 300 vs Class 600What doubling the class does to the flange.Pressure-temperature deratingReading a P-T table and interpolating between temperatures.Flange bolt torqueTarget torque from bolt stress, for every B16.5 bolt size.Choosing a flange face typeRaised face, flat face or ring-type joint, by service.

Common questions

How many bolts does a Class 300 threaded flange use?

NPS 1/2 takes 4 × 1/2 in; NPS 3/4 takes 4 × 5/8 in; NPS 1 takes 4 × 5/8 in; NPS 1 1/4 takes 4 × 5/8 in; NPS 1 1/2 takes 4 × 3/4 in. Bolt count rises with size — see the table above for the full range.

What pressure is a Class 300 flange rated for?

Class is a designation, not a pressure. A Class 300 flange in A105 carbon steel is rated 740 psig (51.0 bar) at 100 °F, and that rating falls as metal temperature rises. In stainless the same class starts lower at ambient but holds up far better in the heat — see the P-T rating tables.

Will a Class 300 flange bolt to a Class 150 flange?

No. Bolt circle diameter, bolt count and bolt size all change with class, so flanges of different classes in the same NPS will not mate. The only exceptions inside B16.5 are the deliberate ones the standard calls out, such as Class 400 sharing Class 600 dimensions in NPS 1/2 through 3 1/2.

Same flange, other classes

Class 150 Class 400 Class 600 Class 900 Class 1500 Class 2500

Same class, other flange types

Weld Neck Slip-On Socket Weld Lap Joint Blind

Every class in one size

NPS 1/2 NPS 3/4 NPS 1 NPS 1 1/4 NPS 1 1/2 NPS 2 NPS 2 1/2 NPS 3 NPS 3 1/2 NPS 4 NPS 5 NPS 6 NPS 8 NPS 10 NPS 12 NPS 14 NPS 16 NPS 18 NPS 20 NPS 24