ASME B16.5 Bore Specification
Inside Diameter (ID) Chart for Weld Neck & Socket Weld Flanges
The inside diameter (bore) of a weld neck flange must precisely match the inside diameter of the mating pipe to prevent flow restrictions, pressure drops, or erosion at the welded butt joint. Below is the full bore reference chart.
Weld Neck Flange Internal Bore (B) Standard Detail
Bore Diameter Alignment with Piping Schedules (STD, XS, XXS)
Complete Weld Neck Flange Inside Diameter Chart (NPS ½" to 24")
| Nominal Size (NPS) | Pipe OD | Sch 10 / 10S Bore ID | Sch 40 / STD Bore ID | Sch 80 / XS Bore ID | Sch 160 Bore ID | XXS Bore ID |
|---|---|---|---|---|---|---|
| 1/2" | 0.840" | 0.674" | 0.622" | 0.546" | 0.464" | 0.252" |
| 3/4" | 1.050" | 0.884" | 0.824" | 0.742" | 0.612" | 0.434" |
| 1" | 1.315" | 1.097" | 1.049" | 0.957" | 0.815" | 0.599" |
| 1-1/4" | 1.660" | 1.442" | 1.380" | 1.278" | 1.160" | 0.896" |
| 1-1/2" | 1.900" | 1.682" | 1.610" | 1.500" | 1.338" | 1.100" |
| 2" | 2.375" | 2.157" | 2.067" | 1.939" | 1.687" | 1.503" |
| 2-1/2" | 2.875" | 2.635" | 2.469" | 2.323" | 2.125" | 1.771" |
| 3" | 3.500" | 3.260" | 3.068" | 2.900" | 2.624" | 2.300" |
| 3-1/2" | 4.000" | 3.760" | 3.548" | 3.364" | -- | 2.728" |
| 4" | 4.500" | 4.260" | 4.026" | 3.826" | 3.438" | 3.152" |
| 5" | 5.563" | 5.295" | 5.047" | 4.813" | 4.313" | 4.063" |
| 6" | 6.625" | 6.357" | 6.065" | 5.761" | 5.187" | 4.897" |
| 8" | 8.625" | 8.329" | 7.981" | 7.625" | 6.813" | 6.875" |
| 10" | 10.750" | 10.420" | 10.020" | 9.750" | 8.500" | 8.750" |
| 12" | 12.750" | 12.390" | 11.938" | 11.374" | 10.126" | 10.750" |
| 14" | 14.000" | 13.624" | 13.250" | 13.000" | 11.188" | -- |
| 16" | 16.000" | 15.624" | 15.250" | 15.000" | 12.876" | -- |
| 18" | 18.000" | 17.624" | 17.250" | 17.000" | 14.438" | -- |
| 20" | 20.000" | 19.564" | 19.250" | 19.000" | 16.064" | -- |
| 24" | 24.000" | 23.500" | 23.250" | 23.000" | 19.312" | -- |
📌 Custom Bore Tolerance Notes
For high-pressure API or custom piping schedules not shown above, specify the exact decimal bore diameter when requesting a quote. Flange bore tolerances conform to ASME B16.5 Section 7.
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