Class 150 Quick Reference

Bolt and bolt-hole diameter are different quantities. For the flange OD, thickness, bolt circle and hole diameter tables, see the Pipe Flange Size Chart. The complete NPS 1/2 to 24 table is in the Class 150 section below.

Bolt/Stud Diameter

The nominal threaded-fastener diameter.

Bolt Count

The total number of bolts or studs around the flange.

Stud Length

The end-to-end length according to the product and joint convention.

Bolt Hole and Bolt Circle

The bolt hole is the clearance hole through the flange, and the bolt circle is the circle through the hole centers. These are different quantities from the bolt diameter.

ASME B16.5-2025 covers pipe flanges and flanged fittings NPS 1/2 through 24 and includes requirements and recommendations on flange bolting, gaskets and joints. Do not extend B16.5 bolting tables above NPS 24.

The reference table used here lists Class 150, 300, 600, 900, 1500 and 2500. Class 400 is also covered by B16.5, but it is not tabulated in that source, so it is not published here.

Class 150 uses 1/2 in. studs up to NPS 1-1/2, then 5/8, 3/4, 7/8, 1, 1-1/8 and 1-1/4 in. as NPS increases to 24. Stud lengths are for 1/16 in. raised-face joints, and RTJ lengths are listed where applicable.

Class 300 usually needs larger diameter studs, more of them, or both. For example, NPS 3/4 uses 4 × 1/2 in. studs in Class 150 and 4 × 5/8 in. studs in Class 300.

Class 150 vs. Class 300 Bolting Examples

Class 150 and Class 300 flanges of the same NPS do not bolt together, because their bolt circles differ even where the bolt count and diameter match.

Class 600 uses the 1/4 in. raised-face stud lengths shown here. Higher classes use the same 1/4 in. raised face, so the stud lengths for RF and RTJ are often the same.

Equal diameter and count at some sizes does not make the classes interchangeable. For example, NPS 1/2 uses 4 × 3/4 in. studs in Classes 900, 1500 and 2500, but the flanges are different. At NPS 2, Classes 900 and 1500 use 8 × 7/8 in. studs and Class 2500 uses 8 × 1 in. studs. Class 2500 stops at NPS 12 under B16.5.

For NPS 4 Class 150 the stud is 5/8 in. (0.625 in.) and the flange hole is 0.75 in., so the hole is larger than the stud:

The clearance is standard-specific. Do not size a flange hole with a generic clearance-hole rule, and do not order 3/4 in. studs because the flange has 3/4 in. holes. For general clearance holes, see the Bolt Hole Size Chart.

The bolt circle is the imaginary circle through the centers of all the bolt holes. For NPS 4 Class 150:

The complete bolt-circle tables are in the Pipe Flange Size Chart, and the Bolt Pattern Calculator can help lay out a pattern. Pipe sizes are in the Pipe Outside Diameter Chart.

Stud Bolt

A fully threaded rod that extends through the joint with a nut on each end. It is standard on most flange connections, especially higher-pressure and critical service.

Machine Bolt

A bolt with an integral hex head and usually one nut. It is used mainly in lower-pressure, non-critical applications. For general bolt dimensions see the Hex Bolt Size Chart and the Bolt Diameter Chart.

Studs do not always have identical lengths, so confirm the length for your joint.

Stud length depends on the combined thickness of the two flanges, the gasket and facing, the nuts, any washers, and enough thread to engage both nuts. Conceptually:

• Ls: required stud length
• T1, T2: flange and joint thickness contributions
• G: compressed gasket contribution
• N1, N2: nut thicknesses
• W: washers, if used
• P: required thread projection or allowance

This is a conceptual stack-up and does not replace B16.5, the project or the manufacturer’s stud-length tables.

RF is raised face and RTJ is ring-type joint. The joint geometry changes the stack, so RF and RTJ do not always use the same stud length. The reference table lists separate lengths where they differ, for example NPS 4 Class 300: 4-1/2 in. for RF and 5 in. for RTJ. At Class 150, RTJ lengths are not listed for NPS 1/2 and 3/4.

Thread size is written as diameter-threads per inch. Think about nominal diameter, threads per inch, the UNC or other specified series, and the thread class. ASME B1.1-2024 is the technical reference. The table lists common UNC designations only; project and bolting specifications govern. See the Thread Size Chart and the UNC Thread Chart.

ASTM A193/A193M covers alloy-steel and stainless-steel bolting materials for pressure vessels, valves, flanges and fittings in high-temperature or high-pressure service, or other special-purpose applications. It lists several grades, ferritic and austenitic. Grade B7 (chromium-molybdenum alloy steel) is a common example, but A193 is not simply “the B7 standard”. ASTM lists A193/A193M-26 as the active edition. Not every flange uses A193 B7.

ASTM A194/A194M-26 covers carbon, alloy, and martensitic and austenitic stainless steel nuts for bolts for high-pressure or high-temperature service, in sizes 1/4 through 4 in. A193 B7 studs are commonly paired with A194 Grade 2H nuts, and A193 B8 studs with Grade 8 nuts, but that pairing is an application and specification choice, not a universal B16.5 requirement. For general nut dimensions see the Nut Size Chart.

Select the studs and nuts together with the flange material, service temperature, pressure, corrosion, coatings or plating, lubrication and the project piping specification. Do not choose a material from the flange class alone.

Washer requirements depend on the joint design, the applicable code or specification, the nut-bearing surface, the assembly procedure and engineering requirements. Not every B16.5 joint requires washers. Where hardened washers are specified, use the specified washer type and not ordinary hardware-store washers. For general dimensions see the Washer Size Chart.

No single torque is correct for a given NPS and class. Torque depends on the stud diameter and material, nut material, lubricant and friction, gasket, target bolt stress, surface condition and the assembly procedure. It is calculated for the joint, and not read from the flange class. For general fastener torque relationships see the Bolt Torque Chart. Pressure-boundary flange torque must also account for the gasket, bolting material, lubrication, target stress and the applicable assembly procedure.

• T: installation torque
• K: nut factor (friction factor)
• D: nominal fastener diameter
• F: desired bolt preload

K can vary a great deal with lubrication, coatings, thread condition and bearing surfaces. This relationship is educational and not a flange torque prescription. The Bolt Torque Calculator applies the general relationship.

ASME PCC-1-2022 covers assembly of pressure-boundary bolted flange joints with ring-type gaskets that sit entirely within the bolt-hole circle, and the guidelines are used to develop effective joint assembly procedures for the broad range of sizes and services. The topics include joint preparation, alignment, gasket installation, bolting condition, controlled tightening in multiple passes and final verification. This page does not reproduce PCC-1 procedure tables.

Do not fully tighten one bolt at a time around the circumference. Use a controlled cross-pattern, multi-pass procedure so the gasket compresses progressively and the bolt load develops more uniformly. The passes below are an illustration of the concept, not the PCC-1 sequence.

B16.5 covers NPS 1/2 to 24 and ASME B16.47 covers NPS 26 to 60, so do not extend this chart to larger flanges. B16.47 has Series A and Series B systems with different bolt patterns, and they must be kept separate by series and class. See the large-diameter section of the Pipe Flange Size Chart.

Limitations