What are the dimensions of an ASME B16.5 flange?
ASME B16.5 fixes outside diameter, thickness, bolt circle diameter, bolt count and bolt size for each combination of nominal size and pressure class. A 6 inch Class 150 flange has a 279.4 mm outside diameter, a 241.3 mm bolt circle and eight bolt holes.
Key Takeaways
- Every dimension of a B16.5 flange is fixed by the combination of nominal size and pressure class.
- Raising the class at the same size increases the outside diameter, the thickness, the bolt circle and usually the bolt count.
- Bolt circle diameter and bolt count are what identify an unmarked flange in the field.
- A 6 inch Class 150 flange takes eight bolts; the same size in Class 300 takes twelve.
- The dimensions below are nominal; tolerances are given in the standard itself.
- Above NPS 24 inch the governing standard becomes ASME B16.47 Series A or B.
The chart
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| NPS | Cl.150 O | Cl.150 C | Cl.150 BCD | Cl.150 bolts | Cl.300 O | Cl.300 C | Cl.300 BCD | Cl.300 bolts |
|---|---|---|---|---|---|---|---|---|
| 1/2" | 88.9 | 11.2 | 60.5 | 4 | 95.2 | 14.2 | 66.7 | 4 |
| 3/4" | 98.6 | 12.7 | 69.9 | 4 | 117.3 | 15.7 | 82.6 | 4 |
| 1" | 107.9 | 14.2 | 79.2 | 4 | 124.0 | 17.5 | 88.9 | 4 |
| 1-1/2" | 127.0 | 17.5 | 98.6 | 4 | 155.4 | 20.6 | 114.3 | 4 |
| 2" | 152.4 | 19.1 | 120.7 | 4 | 165.1 | 22.3 | 127.0 | 8 |
| 3" | 190.5 | 23.9 | 152.4 | 4 | 209.5 | 28.4 | 168.1 | 8 |
| 4" | 228.6 | 23.9 | 190.5 | 8 | 254.0 | 31.8 | 200.0 | 8 |
| 6" | 279.4 | 25.4 | 241.3 | 8 | 317.5 | 36.6 | 269.7 | 12 |
| 8" | 342.9 | 28.4 | 298.5 | 8 | 381.0 | 41.1 | 330.2 | 12 |
| 10" | 406.4 | 30.2 | 361.9 | 12 | 444.5 | 47.8 | 387.4 | 16 |
| 12" | 482.6 | 31.8 | 431.8 | 12 | 520.7 | 50.8 | 450.8 | 16 |
| 14" | 533.4 | 35.0 | 476.3 | 12 | 584.2 | 54.0 | 514.4 | 20 |
| 16" | 596.9 | 36.6 | 539.8 | 16 | 647.7 | 57.2 | 571.5 | 20 |
| 18" | 635.0 | 39.6 | 577.9 | 16 | 711.2 | 60.5 | 628.6 | 24 |
| 20" | 698.5 | 42.9 | 635.0 | 20 | 774.7 | 63.5 | 685.8 | 24 |
| 24" | 812.8 | 47.8 | 749.3 | 20 | 914.4 | 69.9 | 812.8 | 24 |
How to identify an unmarked flange
Markings wear off, paint covers them, and a flange pulled out of a line during a shutdown frequently has nothing legible on the rim. Two measurements identify it: the outside diameter and the bolt circle diameter.
Measure the outside diameter across the rim, then measure the bolt circle — the diameter of the circle passing through the centres of the bolt holes. Count the holes. Those three figures together resolve to one row of the table above, and therefore to one size and class combination.
Bolt count is the fastest discriminator between adjacent classes. A 6 inch Class 150 flange carries eight bolts; the same size in Class 300 carries twelve.
Why higher classes get physically larger
A pressure class is delivered through geometry. Raising the class at a fixed size increases the flange thickness so the disc resists the bending moment from bolt load and internal pressure, increases the outside diameter and bolt circle so the bolts sit further out and generate more clamping moment, and usually increases bolt count so the load distributes more evenly around the gasket.
The practical consequence is spatial. Upgrading a line from Class 300 to Class 600 at design stage is not only a cost change — the flanges are physically bigger and heavier, and on a congested rack that space has to exist. Use the flange weight calculator to see the weight implication before committing.
Frequently Asked Questions
241.3 mm, with eight bolt holes. The flange outside diameter is 279.4 mm and the thickness is 25.4 mm.
Measure the outside diameter and the bolt circle diameter, then count the bolt holes. Those three values together identify the size and pressure class from the dimensional table.
No. The bolt circle diameter and bolt count differ, so the holes do not align. A spool piece or a transition flange is required.
Above NPS 24 inch the governing standard is ASME B16.47, in either Series A or Series B. See our ASME B16.47 flanges page, and confirm which series the project specifies.