What size is a 6 inch Class 150 flange?

11.00 in outside diameter, 9.50 in bolt circle, 8 bolts of 7/8 in, 1.00 in thick. Per ASME B16.5.

What is the bolt circle on a 4 inch flange?

7.50 in, with 8 bolts of 3/4 in, for Class 150. The flange outside diameter is 9.00 in.

How many bolts does an 8 inch flange have?

8 bolts of 7/8 in, on an 11.75 in bolt circle, for Class 150. The outside diameter is 13.50 in.

Does the pressure class change the flange size?

Yes, substantially. A higher class flange at the same pipe size has a larger outside diameter, a larger bolt circle, more or larger bolts and greater thickness. A 6 in Class 300 flange is larger than a 6 in Class 150 flange even though both fit the same pipe.

Can flanges from different manufacturers bolt together?

Yes, if they are the same nominal pipe size, the same pressure class and the same facing type. Dimensional interchangeability is exactly what ASME B16.5 exists to guarantee.

What is the difference between Class 150 and PN16?

They are different standards with different pressure ratings. Class 150 is the ASME imperial system; PN16 is the European (EN 1092) system. They are close in pressure rating but not dimensionally interchangeable — the bolt circles and bolt sizes differ, so the flanges will not bolt together.

Flange Dimension Chart

ASME B16.5 Class 150 flange dimensions — outside diameter, bolt circle, bolt count, bolt hole diameter and flange thickness, from 1/2 in to 24 in.

Data verified 2026-09-29 · based on ASME B16.5-2020

Quick Answer

Flange dimensions are set by pipe size and pressure class, not by pipe size alone. A Class 150 flange on 6 in pipe has a 11.00 in outside diameter and an 8-bolt circle at 9.50 in. Increase the class and the flange gets larger and thicker without the pipe changing.

Find a pipe size

ASME B16.5 Class 150 Flange Dimensions

Pipe size in^[1]	Flange OD in^[1]	Flange OD mm^[2]	Bolt circle in^[1]	Bolts^[1]	Bolt hole in^[1]	Thickness in^[1]
1/2 #	3.50	88.9	2.38	4	0.62	0.38
3/4 #	3.88	98.6	2.75	4	0.62	0.44
1 #	4.25	107.9	3.12	4	0.62	0.50
1 1/4 #	4.62	117.3	3.50	4	0.62	0.56
1 1/2 #	5.00	127.0	3.88	4	0.62	0.62
2 #	6.00	152.4	4.75	4	0.75	0.69
2 1/2 #	7.00	177.8	5.50	4	0.75	0.81
3 #	7.50	190.5	6.00	4	0.75	0.88
3 1/2 #	8.50	215.9	7.00	8	0.75	0.91
4 #	9.00	228.6	7.50	8	0.75	0.94
5 #	10.00	254.0	8.50	8	0.88	0.94
6 #	11.00	279.4	9.50	8	0.88	1.00
8 #	13.50	342.9	11.75	8	0.88	1.12
10 #	16.00	406.4	14.25	12	1.00	1.19
12 #	19.00	482.6	17.00	12	1.00	1.25
14 #	21.00	533.4	18.75	12	1.12	1.38
16 #	23.50	596.9	21.25	16	1.12	1.44
18 #	25.00	635.0	22.75	16	1.12	1.56
20 #	27.50	698.5	25.00	20	1.12	1.69
24 #	32.00	812.8	29.50	20	1.25	1.88

Dimensions are for Class 150 flanges to ASME B16.5. Higher pressure classes (300, 600, 900, 1500, 2500) use larger and thicker flanges at the same pipe size. Bolt hole diameter is given for the standard UNC bolt series; stud bolt length depends on the assembly.

Pipe Size, Flange Size and Pressure Class

Three different quantities are involved, and mixing them up is the most common error in flange specification:

Nominal pipe size — the pipe it fits, in inches. It is a nominal label: a 6 in pipe does not measure 6 in in any dimension.
Pressure class — 150, 300, 600, 900, 1500 or 2500. It sets how much pressure the joint is rated for, and therefore how heavy the flange is.
Flange outside diameter and thickness — the result of the first two. A 6 in Class 150 flange is 11.00 in across; the Class 300 flange for the same pipe is larger again.

So a flange is never specified by pipe size alone. The class must always be stated, because the flange for a given pipe size exists in six different sizes.

Bolt Circle and Bolt Count

The bolt circle is the diameter on which the bolt holes are centred, and it must match between the two flanges being joined. Because bolt circle is fixed by pipe size and class, flanges of the same size and class from different manufacturers will always bolt together — that is the point of the standard.

Bolt count jumps in steps rather than growing smoothly: 1/2 to 3 in uses 4 bolts, then 3 1/2 to 8 in uses 8, 10 and 12 in use 12, and 16 and 18 in use 16. This is why the number of bolts is not a reliable way to identify a flange — several sizes share the same count.

Frequently Asked Questions

What size is a 6 inch Class 150 flange?: 11.00 in outside diameter, 9.50 in bolt circle, 8 bolts of 7/8 in, 1.00 in thick. Per ASME B16.5.
What is the bolt circle on a 4 inch flange?: 7.50 in, with 8 bolts of 3/4 in, for Class 150. The flange outside diameter is 9.00 in.
How many bolts does an 8 inch flange have?: 8 bolts of 7/8 in, on an 11.75 in bolt circle, for Class 150. The outside diameter is 13.50 in.
Does the pressure class change the flange size?: Yes, substantially. A higher class flange at the same pipe size has a larger outside diameter, a larger bolt circle, more or larger bolts and greater thickness. A 6 in Class 300 flange is larger than a 6 in Class 150 flange even though both fit the same pipe.
Can flanges from different manufacturers bolt together?: Yes, if they are the same nominal pipe size, the same pressure class and the same facing type. Dimensional interchangeability is exactly what ASME B16.5 exists to guarantee.
What is the difference between Class 150 and PN16?: They are different standards with different pressure ratings. Class 150 is the ASME imperial system; PN16 is the European (EN 1092) system. They are close in pressure rating but not dimensionally interchangeable — the bolt circles and bolt sizes differ, so the flanges will not bolt together.

Value Sources

Each data column on this page is tied to the source it came from. The numbers in square brackets correspond to the table headers above.

#	Source	Type	Revision / method
[1]	ASME B16.5 — Pipe Flanges and Flanged Fittings (NPS 1/2 through NPS 24)	standard	ASME B16.5-2020 — source
[2]	Millimetre conversion	derived	Inch value × 25.4.

Data Sources

Standard	Revision	What it covers on this page
ASME B16.5 — Pipe Flanges and Flanged Fittings	ASME B16.5-2020	all Class 150 dimensions on this page

Cross-checked against:

Outside diameters spot-verified at build time against ASME B16.5 tabulated values for 1 in, 6 in and 12 in
Millimetre column verified at build time against the inch column

Derived values — the following values on this page are calculated, not taken directly from the standard:

Value	How it is derived
Millimetre outside diameter	Inch value × 25.4.

Class 150 only. Higher pressure classes — 300, 600, 900, 1500 and 2500 — have different dimensions throughout and are published separately in the same standard. Verify facing type (raised face, flat face, ring joint) and gasket selection separately.

Accuracy and use. The values on this page are compiled from the published standards and cross-checked sources listed above. Where values are derived, the derivation is stated. No warranty, express or implied, is made as to the accuracy or completeness of this information, and no liability is accepted for any loss or damage arising from its use. Engineering reference data is provided for guidance in preliminary work — before a value is used for design, fabrication or acceptance testing, verify it against the current revision of the governing standard and against your own inspection. The user assumes all risk and responsibility in connection with the use of this information.

Cite This Page

Every value on this page is traceable to the sources listed above. If you use the data in a document, paper or report, cite it as:

Plain text

Engineering Reference. "Flange Dimensions." https://refmanual.com/charts/flange-dimensions/ (accessed 2026-09-29). Data from ASME B16.5 — Pipe Flanges and Flanged Fittings, ASME B16.5-2020.

BibTeX

@misc{flange-dimensions,
  title        = {Flange Dimensions},
  author       = {{Engineering Reference}},
  howpublished = {\url{https://refmanual.com/charts/flange-dimensions/}},
  note         = {Accessed: 2026-09-29},
  source       = {ASME B16.5 — Pipe Flanges and Flanged Fittings, ASME B16.5-2020}
}

RIS (EndNote, Zotero, Mendeley)

TY  - ELEC
TI  - Flange Dimensions
AU  - Engineering Reference
PB  - Engineering Reference
UR  - https://refmanual.com/charts/flange-dimensions/
PY  - 2026
DA  - 2026-09-29
N1  - Source: ASME B16.5 — Pipe Flanges and Flanged Fittings, ASME B16.5-2020
ER  -

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