Flange Bolt-Pattern & Dimension Chart (ASME B16.5 Class 150)
This chart gives the bolt circle diameter, number of bolts, and bolt size for ASME B16.5 Class 150 flanges. Because Class 150 and iron ASME B16.1 Class 125 share the same drilling, the same pattern applies to most ductile-iron valves and companion flanges.
Class 150 / Class 125 bolt pattern
Verify against the current ASME standard for critical or high-pressure work.
| Nominal pipe size | Bolt circle (in) | No. of bolts | Bolt diameter (in) |
|---|---|---|---|
| 1/2" | 2.38 | 4 | 1/2 |
| 3/4" | 2.75 | 4 | 1/2 |
| 1" | 3.12 | 4 | 1/2 |
| 1-1/4" | 3.50 | 4 | 1/2 |
| 1-1/2" | 3.88 | 4 | 1/2 |
| 2" | 4.75 | 4 | 5/8 |
| 2-1/2" | 5.50 | 4 | 5/8 |
| 3" | 6.00 | 4 | 5/8 |
| 4" | 7.50 | 8 | 5/8 |
| 5" | 8.50 | 8 | 3/4 |
| 6" | 9.50 | 8 | 3/4 |
| 8" | 11.75 | 8 | 3/4 |
| 10" | 14.25 | 12 | 7/8 |
| 12" | 17.00 | 12 | 7/8 |
How to use it
Match the bolt circle and bolt count to confirm two flanges mate. Remember Class 150 and Class 300 are not interchangeable — Class 300 uses a different (larger) pattern. Always tighten flange bolts in a star/criss-cross sequence in stages for even gasket compression.
Related
See the flange selection guide for types and pressure classes, and browse flanges.
Frequently Asked Questions
How do I measure a flange bolt circle in the field?
The bolt circle diameter (BCD) is the diameter of the imaginary circle that passes through the center of each bolt hole. For a 4-bolt flange, measure center-to-center across any two opposite holes — that distance is the bolt circle diameter. For flanges with 8 or more holes, measure between opposite hole centers for the most accurate reading, then cross-check against the chart values for your nominal pipe size.
How many bolts does a Class 150 flange have, and does it change by size?
Yes, bolt count increases with pipe size. Sizes 1/2" through 3" use 4 bolts; 4" through 8" jump to 8 bolts; 10" and 12" use 12 bolts. The bolt diameter also steps up with size — from 1/2" bolts on small sizes to 7/8" bolts on 10"-12" flanges. The full table on this page lists all values from 1/2" through 12".
What is the difference between ASME B16.5 and ASME B16.1 drilling?
ASME B16.5 governs steel and alloy flanges in pressure classes 150 through 2500. ASME B16.1 governs cast-iron flanges at Class 125 and 250. Importantly, B16.5 Class 150 and B16.1 Class 125 share the same bolt circle and hole count for matching nominal sizes — so a steel Class 150 flange mates directly to a cast-iron or ductile-iron Class 125 valve without an adapter. Class 300 drilling is different and is not interchangeable with 150/125.
What gasket should I use with Class 150 flanges?
For raised-face (RF) flanges, use a ring gasket sized to sit between the raised faces — spiral wound, ring joint, or flat sheet gaskets in the appropriate material for your service (rubber for water, graphite or PTFE for chemicals, metal ring joint for high-pressure/high-temp). For flat-face (FF) flanges mating to cast iron, use a full-face gasket that covers the entire flange face to distribute the bolt load evenly. Always match the gasket material to the fluid and temperature.