A flange dimension table is not complicated, but the columns and footnotes confuse anyone seeing one for the first time. Once a few basics land, the table reads quickly.
What the columns say
| Column | Meaning | Why it is needed |
|---|---|---|
| NPS / DN | Nominal size | The nominal size of the line, not the real diameter |
| O | Flange outside diameter | Space check and mating flange compatibility |
| C | Flange thickness | Weight and stud length calculation |
| BC | Bolt circle | The real determinant of compatibility |
| No. / dia. | Bolt count and hole diameter | Stud count and diameter |
| X | Hub diameter | For weld neck and slip-on types |
| B | Bore | Varies with pipe schedule |
| Y | Overall height | From pipe end to flange face |
The key point: at the same nominal size, O, C and BC all change with pressure class. "A 4 inch flange" alone therefore gives no dimension.
Why the bore is not a single value
The bore of a weld neck flange is machined to the wall thickness of the pipe it joins. Tables therefore list values by schedule or carry a note saying "to suit pipe schedule". That is why the schedule must be stated when ordering.
ASME versus EN tables
ASME tables are inch based and give Class ratings. EN 1092-1 tables are metric and use DN and PN. At the same nominal size the bolt circle and bolt count differ, so the tables are not interchangeable.
Using the table
- Confirm the standard and class of the mating flange
- Calculate stud length from two flange thicknesses, gasket thickness and nut heights
- Use the weight column, where given, for transport planning
- Remember that thickness and height change with type (WN, SO, blind, LJ)
Our flange dimension tables are organised by type and class under ASME B16.5 and EN 1092-1. Send your list through the enquiry form.
FAQ
Is the nominal size the real dimension?
No. NPS and DN are nominal designations; the real outside diameter is given in its own column.
Why do two flanges of the same size not fit?
Because the pressure class or standard differs, which changes the bolt circle and bolt count.
Why does the weld neck bore vary?
The hub bore is machined to the pipe wall thickness, so it depends on the schedule.
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