How to Determine the Number of Bolt Holes in a Flange

A steel flange is an annular plate that joins pipe, valves and equipment with bolts and a seal, so the joint can be taken apart for maintenance. The number of bolt holes is not a free design choice: for catalog components it is fixed by the pressure class and the nominal size in the standard tables, and that is what keeps standard flange gaskets interchangeable across projects and suppliers.

Flange Standard, Type and Pressure Class

Different standards (ASME B16.5, EN 1092-1, HG/T 20592-20635) tabulate the number of bolt holes, the hole diameter and the bolt circle diameter for every pressure class and nominal size, and that table is the primary basis for the design. Flange type — weld neck, slip-on, threaded, blind or socket weld — follows the same table, so the hole pattern does not change with the manufacturing form.

• Pressure class (Class 150-2500 or PN 6-160) sets the bolt load: higher classes need more bolts, or larger bolts, to build the clamping force the seal requires.
• Design temperature matters because heat lowers bolt strength and creates differential expansion between flange and bolts; hot service often takes more bolts for even clamping.
• Nominal size (DN / NPS) sets the bolt circle circumference: a larger circle carries more holes at the same minimum spacing.

Diameter, Bolt Circle and Structural Rigidity

The bolt circle grows with the nominal size, so large pipe flanges naturally take more bolts at the same spacing. Thickness and hub geometry add stiffness: a thin or flexible plate needs more bolts to limit rotation and to load the gasket evenly, otherwise the seal crushes next to the bolts and goes loose between them. Standard tables already combine these effects, which is why the hole count is read from the table rather than calculated for catalog components.

Table 1. Factors that fix the bolt hole count of a standard flange; the final value comes from the standard table for the chosen class and size.

Sealing Face, Bolt Selection and Field Practice

The sealing face decides how clamping force is distributed: raised face (RF), male-female (MFM), tongue-and-groove (TG) and ring joint (RJ) each have their own rules for hole number and hole size, and ring joint flanges used in high-pressure, high-temperature service are regulated most strictly. Bolt strength and gasket width then set how many holes are actually needed.

• Gasket width and rigidity: wide or soft gaskets need dense, even clamping, which pushes the design toward more bolts.
• Bolt strength and size: high-strength, large-diameter studs, for example ASTM A193 B7 with ASTM A194 2H nuts, allow fewer holes for the same total load; smaller or lower-grade bolts need more.
• Tool access and external loads: bolt spacing must leave room for socket and torque wrench, and bending, shear or vibration may require extra bolts or a different pattern.

In practice the designer follows a fixed sequence: define the medium, design pressure and design temperature; choose the standard, sealing face type and gasket; set the pressure class and size; calculate the total bolt load from internal pressure plus gasket seating; then select bolt material and size and take the standard hole pattern that meets the load. Detailed calculation is only needed for non-standard flanges or special conditions.

Frequently Asked Questions

How many bolt holes does a Class 150 flange have?

It depends on the nominal size. For standard flanges the number is read from the standard table for the given pressure class and DN / NPS: an NPS 4 (DN 100) Class 150 flange, for example, has 8 bolt holes.

Does the bolt hole count change with the flange type?

No. For one pressure class and size, weld neck, slip-on, threaded, blind and socket weld flanges share the same hole pattern from the same standard table.

Why do high-pressure flanges have more bolt holes?

A higher class needs a greater total clamping force, and that force is built from more bolts, larger bolts, or both, so the hole count and the bolt diameter rise together.

When is a non-standard bolt hole pattern justified?

Only for non-standard flanges or special conditions: large external loads, unusual temperatures or special gaskets. In every other case the standard table is the faster and safer answer.