Threaded Flange Dimensions

Tapped with an ASME B1.20.1 NPT taper thread and screwed onto matching pipe, with no welding at all. That makes it the flange for places where a hot work permit is impossible.

Standard: ASME B16.5
Abbreviation: THD
Pressure classes: 7
Size range: NPS 1/2 – NPS 24
Typical service: Low-pressure utility piping and hazardous areas where welding is banned.

Threaded Flange in section, and a Class 150 flange from the front. The section shows what each column of the dimension tables measures, with welds in orange; it is not to scale. The front view is an NPS 6 Class 150 flange drawn to scale, with its bolt holes straddling the centrelines.

Dimensions by pressure class

Class 150NPS 1/2 – 2420 sizes Class 300NPS 1/2 – 2420 sizes Class 400NPS 1/2 – 2420 sizes Class 600NPS 1/2 – 2420 sizes Class 900NPS 1/2 – 2419 sizes Class 1500NPS 1/2 – 2 1/27 sizes Class 2500NPS 1/2 – 2 1/27 sizes

Strengths

No welding, so usable in flammable or explosive areas
Fast field installation with hand tools

Limitations

Not for cyclic, high-temperature or vibrating service — the thread loosens
Thread root reduces the effective wall; needs thick-wall pipe

Class 150 dimensions

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

ASME B16.5 Class 150 threaded flange dimensions — the most commonly specified class.
Size	Flange OD	Thickness	Bolt circle	Bolts	Bolt dia.	Raised face OD
NPS 1/2	3.5088.90 mm	0.389.65 mm	2.3860.45 mm	4	1/2 in	1.3835.05 mm
NPS 3/4	3.8898.55 mm	0.4411.18 mm	2.7569.85 mm	4	1/2 in	1.6942.93 mm
NPS 1	4.25107.95 mm	0.5012.70 mm	3.1279.25 mm	4	1/2 in	2.0050.80 mm
NPS 1 1/4	4.62117.35 mm	0.5614.22 mm	3.5088.90 mm	4	1/2 in	2.5063.50 mm
NPS 1 1/2	5.00127.00 mm	0.6215.75 mm	3.8898.55 mm	4	1/2 in	2.8873.15 mm
NPS 2	6.00152.40 mm	0.6917.53 mm	4.75120.65 mm	4	5/8 in	3.6291.95 mm
NPS 2 1/2	7.00177.80 mm	0.8120.57 mm	5.50139.70 mm	4	5/8 in	4.12104.65 mm
NPS 3	7.50190.50 mm	0.8822.35 mm	6.00152.40 mm	4	5/8 in	5.00127.00 mm
NPS 3 1/2	8.50215.90 mm	0.8822.35 mm	7.00177.80 mm	8	5/8 in	5.50139.70 mm
NPS 4	9.00228.60 mm	0.8822.35 mm	7.50190.50 mm	8	5/8 in	6.19157.23 mm
NPS 5	10.00254.00 mm	0.8822.35 mm	8.50215.90 mm	8	3/4 in	7.31185.67 mm
NPS 6	11.00279.40 mm	0.9423.88 mm	9.50241.30 mm	8	3/4 in	8.50215.90 mm
NPS 8	13.50342.90 mm	1.0626.92 mm	11.75298.45 mm	8	3/4 in	10.62269.75 mm
NPS 10	16.00406.40 mm	1.1228.45 mm	14.25361.95 mm	12	7/8 in	12.75323.85 mm
NPS 12	19.00482.60 mm	1.1930.23 mm	17.00431.80 mm	12	7/8 in	15.00381.00 mm
NPS 14	21.00533.40 mm	1.3133.27 mm	18.75476.25 mm	12	1 in	16.25412.75 mm
NPS 16	23.50596.90 mm	1.3835.05 mm	21.25539.75 mm	16	1 in	18.50469.90 mm
NPS 18	25.00635.00 mm	1.5038.10 mm	22.75577.85 mm	16	1 1/8 in	21.00533.40 mm
NPS 20	27.50698.50 mm	1.6241.15 mm	25.00635.00 mm	20	1 1/8 in	23.00584.20 mm
NPS 24	32.00812.80 mm	1.8145.97 mm	29.50749.30 mm	20	1 1/4 in	27.25692.15 mm

Higher classes are on the Class 300 page and above.

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.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.Pressure-temperature deratingReading a P-T table and interpolating between temperatures.

Common questions

When should I use a threaded flange?

Low-pressure utility piping and hazardous areas where welding is banned.

What pressure classes does a threaded flange come in?

ASME B16.5 publishes the threaded flange in Class 150, Class 300, Class 400, Class 600, Class 900, Class 1500 and Class 2500, in sizes NPS 1/2 through NPS 2 1/2 in Classes 1500 and 2500, and through NPS 24 in the other classes. Above NPS 24 large-diameter flanges are covered by ASME B16.47 instead.

Does the flange face type change these dimensions?

The flange outside diameter, bolt circle and bolting are the same whatever the face. What changes is the facing itself and the flange thickness measurement: B16.5 thickness excludes the raised face, so a Class 300 raised-face flange is 0.06 in thicker overall than the tabulated figure. Ring-type joint flanges are grooved instead and are dimensioned separately.