How to Measure a Flange

What it is

Flanges of different sizes and pressure classes look alike. Measuring a few key dimensions lets you find the nominal pipe size (NPS), pressure class and standard. Small and mid-size flanges (NPS 1/2 through 24) are covered by ASME B16.5. Large diameter flanges (NPS 26 through 60) are covered by ASME B16.47 Series A or Series B, which are not interchangeable.

How to measure

1. Outside diameter (OD): measure across the full flange.
2. Number of bolt holes: count them.
3. Bolt hole diameter: measure one hole.
4. Bolt circle (BC): measure center to center across opposite holes. With an even number of holes, measure from the inside edge of one hole to the outside edge of the hole directly across; this equals the bolt circle.
5. Thickness: measure the flange ring, not including the raised face.
6. Facing: raised face (RF), flat face (FF) or ring type joint (RTJ) groove. See Flange Facings (RF, FF, RTJ).
7. Bore and hub: measure the bore and note the type (weld neck, slip-on, socket, threaded, blind, lap joint).
8. Read the markings: forged flanges are usually stamped with size, class, material and standard. See Fitting and Flange Marking Chart.

Measurement checklist

Bolt circle from hole spacing

If only adjacent holes can be reached, the bolt circle equals the adjacent center-to-center distance divided by sin(180° ÷ number of holes). For 4 holes, multiply the adjacent spacing by 1.414. For 8 holes, divide the adjacent spacing by 0.3827.

Key points

• Compare all measurements against the tables. One dimension alone is not enough.
• See Flange Bolt Pattern Lookup Guide and Flange Class Identification Chart.
• Pressure-temperature ratings depend on class and material. See Pressure Class & Flange Ratings Explained.

Common mistakes

• Measuring thickness over the raised face.
• Confusing B16.47 Series A and Series B.
• Assuming a flange is ASME when it may be EN, JIS or AWWA.

Governing standards: ASME B16.5 (pipe flanges NPS 1/2–24); ASME B16.47 (large diameter flanges NPS 26–60).

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.