Sources and scope for this page: ASME B16.5-2020, covering all Class 150 dimensions on this page. No independent source check is recorded for this page: the content rests on the sources listed and on the working shown on the page. Full sources, values and limits on this page.
Page last changed 2026-10-19 · based on ASME B16.5-2020
Quick Answer
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.
Related
Value Sources
References and calculation methods used for this page are listed below. Numbered markers connect table columns where present. A reference listing does not itself establish that every value has been independently checked.
| # | Source | Type | Revision / method |
|---|---|---|---|
| [1] | ASME B16.5 — Pipe Flanges and Flanged Fittings (NPS 1/2 through NPS 24) | reference | 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 |
Verification scope
No page-specific independent source review is recorded here. The listed references and formulas explain the basis of the compilation; automated consistency checks do not verify the original sources or establish product conformity.
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.
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