Flange Dimensions — ASME B16.5 Class 150 & 300
Flange dimensions per ASME B16.5 for Class 150 and Class 300, sizes 1 in to 24 in: outside diameter, bolt circle, bolt count and size, and flange thickness.
| Size (in) | Class | Outside dia. (in) | Bolt circle (in) | No. of bolts | Bolt size (in) | Thickness (in) |
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Standard: ASME B16.5 · Data checked: 2026-10-14
Frequently asked questions
How do I work out the bolt length for a flange?
Bolt length is not on the flange dimension table because it depends on the gasket thickness and whether you are using a stud bolt with two nuts or a bolt with one nut. Total length is flange thicknesses, plus gasket, plus nut and washer stack. Calculate it per joint.
What is the difference between Class 150 and Class 300?
Pressure rating and dimensions: Class 150 is roughly 285 psi at ambient, Class 300 about 740 psi, and the higher class also has a larger bolt circle, more bolts and a thicker flange. They are not interchangeable even at the same nominal size.
Can I mix flange classes in a joint?
No. A Class 150 and a Class 300 flange of the same size have different bolt circles and different thicknesses, so they will not make up correctly. Both flanges in a joint must be the same class and the same facing type.
Why are there more bolts on larger sizes?
Because the bolt circle grows and the bolt spacing has to stay tight enough to compress the gasket evenly. Spacing that is too wide lets the gasket relax between bolts, which is where leaks start.