Flange Bolt Pattern Lookup Guide

What it is

A flange bolt pattern is the number of bolt holes, the hole size, and the bolt circle diameter: the diameter of the circle through the centers of the holes. Bolt patterns are fixed by the flange standard, the pipe size and the class. ASME B16.5 covers NPS 1/2 through 24. ASME B16.47 covers NPS 26 through 60 in two different series, Series A and Series B, which do not share patterns.

How to use it

1. Identify the standard (B16.5 or B16.47 Series A or B).
2. Identify the nominal pipe size.
3. Identify the class from the stamp.
4. Look up the size and class in the standard's dimension table. It lists flange OD, bolt circle, number of holes, hole diameter and bolt diameter.
5. If the flange is unmarked, measure the bolt circle as below and compare to the tables.

Measuring an unknown bolt circle

For an even number of holes, measure from the center of one hole to the center of the hole straight across. That distance is the bolt circle.

For a quick check from two adjacent holes, measure center to center and multiply by the factor below. Bolt circle = adjacent spacing × factor.

The factor is 1 divided by the sine of (180° ÷ number of holes).

Key points

• ASME flange bolt holes come in multiples of four and straddle the vertical and horizontal centerlines. No hole sits on the centerline.
• Bolt holes are larger than the bolt. The table gives both. Do not size bolts from the hole.
• Many sizes in Class 150 and 300 share some dimensions but not the full pattern. Check every value.
• Class 125 and 250 cast iron flanges (ASME B16.1) share bolt patterns with some Class 150 and 300 steel flanges. Confirm before mixing.
• Stud length depends on flange thickness, gasket and nuts. See the Stud Bolt Length Lookup Guide and the governing tables.

Common mistakes

• Measuring from the edge of one hole to the far edge of another and calling it the bolt circle.
• Assuming a B16.47 Series A flange matches Series B at the same size.
• Ordering bolts by hole size, which gives oversized bolts.
• Assuming two flanges with the same OD have the same pattern.

Governing standards: ASME B16.5 (NPS 1/2 through 24); ASME B16.47 Series A and B (NPS 26 through 60); ASME B16.1 (cast iron flanges).

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.