Free engineering tool
Flange Identifier: Size and Class From Your Measurements
Got a flange with no readable markings? Measure four things: the outside diameter, the bolt circle (center of one hole to the center of the hole straight across), the number of bolt holes and the bolt hole diameter. This identifier compares them with every ASME B16.5 size and class (1/2 to 24 inch, Class 150 to 2500) and ASME B16.47 Series A and B (26 to 60 inch) on the Texas Flange flange dimension tables, and ranks the closest matches. The bolt circle and hole count matter most. Only have the spacing between two neighboring holes? Turn it into a bolt circle with the bolt circle calculator.
Edge to edge, across the widest point.
Center of one hole to the center of the hole straight across.
Only have the spacing between two neighboring holes? Turn it into a bolt circle
Reference tool for planning and quoting. Confirm against the purchase specification and your site's procedures before use.
Request a flange quote
Frequently asked questions
How do you measure a flange size?
Measure the outside diameter, the bolt circle diameter (center of one bolt hole to the center of the hole directly across), the number of bolt holes and the bolt hole diameter. The bolt circle and hole count identify the size and pressure class far better than the outside diameter alone.
Is the flange size the same as the outside diameter?
No. The nominal size (NPS) refers to the pipe the flange fits. A 4 inch Class 150 flange, for example, has a 9 inch outside diameter and a 7-1/2 inch bolt circle with eight 3/4 inch holes.
Why do some results show two classes?
Some sizes share the same outside diameter and drilling in two classes. On ASME B16.5, for example, Class 300, 400 and 600 share the same O.D. and drilling at 1/2 inch, and Class 900 is identical to Class 1500 from 1/2 to 2-1/2 inch. Check the markings or hub, or use the higher class.
How do I measure the bolt circle on a flange with an odd number of holes?
Measure the center-to-center spacing between two neighboring holes and divide by sin(180 / number of holes), or enter the spacing in the Texas Flange bolt circle calculator.
What if my flange does not match any ASME size?
It may be to another standard, such as AWWA, MSS SP-44, EN 1092-1 (DIN) or JIS, or it may be custom-drilled. Send your measurements to Texas Flange and we will help you find a match.
