ASME B16.5 Standard: Guide to Flange Sizes, Classes and Requirements

ASME B16.5 is a widely used piping standard covering pipe flanges and flanged fittings. It sets requirements for pressure-temperature ratings, materials, dimensions, tolerances, marking, testing, and opening designations. For engineers, piping contractors, distributors and purchasing teams, understanding this standard is an important first step in specifying compatible flanges for industrial piping systems.

However, specifying a flange to ASME B16.5 involves more than choosing a nominal pipe size or a pressure class. You must also consider the applicable material, temperature, flange type, facing, dimensional requirements, and project specifications.

This guide explains what ASME B16.5 covers, how its size range and pressure classes are organized, and what buyers should check before specifying or ordering flanges.

What Is ASME B16.5?

ASME B16.5 Pipe Flanges and Flanged Fittings: NPS 1/2 Through NPS 24, Metric/Inch Standard, is a standard published by the American Society of Mechanical Engineers (ASME).

The standard addresses several aspects of pipe flanges and flanged fittings, including:

• Pressure-temperature ratings
• Materials
• Dimensions and tolerances
• Marking and testing
• Methods of designating openings
• Requirements and recommendations concerning flange bolting, gaskets and joints

It is used in conjunction with other applicable piping and equipment standards. Compliance with ASME B16.5 alone does not establish that a flange is suitable for every service condition. Material selection, the applicable piping code, operating conditions and project requirements must also be considered.

The current published edition listed by ASME is ASME B16.5-2025. Buyers and engineers should nevertheless confirm which edition is specified in the contract, purchase order or governing project documents before placing an order.

What Does ASME B16.5 Cover?

ASME B16.5 covers pipe flanges and flanged fittings within defined size and rating ranges. It also addresses selected blind flanges and reducing flanges made from the materials permitted by the standard.

Its scope includes both dimensional requirements and pressure-temperature ratings. These are related, but they serve different purposes.

Dimensional requirements establish the relevant physical characteristics of a flange, such as its outside diameter, thickness, bolt-hole arrangement and facing dimensions. Pressure-temperature ratings establish the allowable pressure for a given rating class, material and temperature under the standard.

A flange that matches the required nominal pipe size may still be unsuitable if its class, material, facing or bolt-hole arrangement does not match the mating component.

ASME B16.5 should therefore be treated as a coordinated set of requirements rather than simply a flange dimension chart.

ASME B16.5 Sizes: What Is the Standard Size Range?

The title of ASME B16.5 identifies its general nominal pipe size range as NPS 1/2 through NPS 24. However, the applicable maximum size depends on the type of component and its pressure class.

For flanges, the standard covers rating classes 150, 300, 400, 600, 900 and 1500 from NPS 1/2 through NPS 24. Class 2500 flanges are covered from NPS 1/2 through NPS 12.

The scope for flanged fittings differs in some respects. Classes 150 and 300 are covered from NPS 1/2 through NPS 24. The standard also identifies higher-rated flanged fittings within specified ranges and under the conditions described in its scope and appendices.

These distinctions matter when reviewing a drawing or purchasing specification. The general size range in the standard’s title should not be interpreted as meaning that every class and every component type is covered at every size.

What Does NPS Mean in ASME B16.5?

NPS stands for Nominal Pipe Size. It is a nominal designation used to identify pipe sizes and related piping components.

NPS is not necessarily the measured inside diameter or outside diameter of a flange. The actual dimensions of a flange must be obtained from the applicable standard tables and must correspond to the required size, class and flange type.

For example, identifying a flange as NPS 6 does not provide enough information to establish its complete dimensional specification. Its pressure class, facing, material and other applicable requirements still need to be identified.

Are All ASME B16.5 Flanges the Same Size at a Given NPS?

No. NPS identifies the nominal size, but other dimensions can vary with the flange type and pressure class.

Depending on the component and applicable requirements, relevant dimensions may include:

• Flange outside diameter
• Flange thickness
• Bolt circle diameter
• Number and diameter of bolt holes
• Raised-face or other facing dimensions
• Bore and hub dimensions

When checking compatibility with existing piping, compare the applicable dimensional requirements rather than relying on the nominal size alone.

For a detailed dimensional reference, see our ASME B16.5 flange dimensions and selection guide.

ASME B16.5 Classes Explained

ASME B16.5 uses pressure class designations to identify pressure-temperature rating categories. The principal flange classes are:

This table summarizes the flange size ranges specified in the standard. It is not a pressure-rating table and does not mean that every listed class can be used at the same pressure or temperature.

Does Class 150 Mean 150 PSI?

No. The number in an ASME pressure class designation is not a direct statement of the flange’s allowable working pressure in psi.

For example, a Class 150 flange does not automatically have a maximum allowable working pressure of 150 psi under every condition.

The applicable pressure-temperature rating depends on factors including the material group, operating temperature and the relevant rating tables in the applicable edition of ASME B16.5.

For this reason, Class 150, Class 300 and other designations should not be converted into fixed pressure values without checking the appropriate standard requirements.

What Is the Difference Between Class 150 and Class 300?

Class 150 and Class 300 are different pressure class designations. For a given nominal size and flange type, they may have different dimensional requirements and pressure-temperature ratings.

Class 300 should not be interpreted as having exactly twice the allowable pressure of Class 150. The applicable rating must be determined from the standard’s pressure-temperature tables for the relevant material and operating temperature.

When selecting between the two classes, engineers should consider the design conditions and the governing piping code rather than choosing solely on the basis of the class number.

How Do You Determine the Allowable Pressure?

Use the pressure-temperature rating provisions of the applicable edition of ASME B16.5.

The basic process is to:

1. Identify the applicable flange class.
2. Confirm the material specification and the corresponding material group.
3. Establish the design temperature.
4. Consult the applicable pressure-temperature rating table.
5. Confirm that the resulting allowable pressure is suitable for the design conditions and applicable piping code.

A class designation alone is not sufficient to approve a flange for a particular operating condition. Where the service involves demanding temperatures, cyclic loading, corrosive media or other special conditions, additional engineering requirements may apply.

ASME B16.5 Materials

ASME B16.5 addresses materials used for covered flanges and flanged fittings. The suitability of a particular material depends on the component specification, the applicable material group and the service conditions.

Common material specifications encountered in industrial flange procurement include ASTM A105 for carbon steel forgings, ASTM A350 LF2 for specified low-temperature carbon steel forgings, and ASTM A182 grades for covered alloy and stainless steel forgings.

These material specifications are not interchangeable. Each has its own chemical, mechanical and application requirements, and the applicable product specification must be checked before a material is selected.

When reviewing a flange purchase order, confirm:

• The required material specification and grade
• The required pressure class and nominal size
• The operating temperature and service conditions
• Any project-specific impact testing or supplementary requirements
• Required inspection records, material test reports and traceability documentation

ASME B16.5 does not replace the material specification. The standard and the applicable material specification work together to define the requirements for the ordered component.

ASME B16.5 vs. ASME B16.47: What Is the Difference?

ASME B16.5 and ASME B16.47 are separate standards covering pipe flanges within their respective scopes.

ASME B16.5 covers flanges through NPS 24, subject to the class-specific ranges described above. ASME B16.47 addresses large-diameter steel flanges in its own specified scope.

For a project involving large-diameter flanges, the appropriate standard should be determined from the design documents, required size, rating, flange arrangement and applicable project requirements.

Do not assume that a flange is interchangeable simply because its nominal size and pressure class appear similar. The dimensional requirements and other applicable specifications must be checked.

For a more detailed comparison, read our guide to ASME B16.47 vs. B16.5 flange standards.

What Should Buyers Check Before Ordering ASME B16.5 Flanges?

For distributors, stockists, contractors and industrial purchasing teams, a clear technical specification helps reduce avoidable clarification and compatibility problems.

Before requesting a quotation, prepare the following information where applicable:

Not every project requires the same supplementary documents or inspections. The purchase order and project specification should identify which requirements apply.

If the flange will connect to existing equipment or piping, provide the mating flange details or approved drawings when possible. This helps the supplier check the specified dimensions and connection requirements before production.

Frequently Asked Questions About ASME B16.5

What is ASME B16.5 used for?

ASME B16.5 specifies requirements for covered pipe flanges and flanged fittings, including pressure-temperature ratings, materials, dimensions, tolerances, marking, testing and methods of designating openings. It is used when specifying and verifying components for piping and related equipment.

What sizes are covered by ASME B16.5?

The standard’s general title range is NPS 1/2 through NPS 24. However, the permitted range depends on the component type and class. For example, Class 2500 flanges are covered through NPS 12, while the other principal flange classes listed in the standard extend through NPS 24.

What are the ASME B16.5 pressure classes?

The principal flange class designations are 150, 300, 400, 600, 900, 1500 and 2500. The appropriate class depends on the applicable pressure-temperature rating, material and design requirements.

Does ASME B16.5 define flange dimensions?

Yes. The standard covers dimensions and tolerances for the flanges and flanged fittings within its scope. The relevant dimensions depend on the specified size, class, component type and other applicable requirements.

Is ASME B16.5 the same as ANSI B16.5?

The designation “ANSI B16.5” is commonly encountered in commercial descriptions and older references. For current procurement, identify the applicable ASME B16.5 edition and the exact requirements stated in the project documents rather than relying on a shorthand description alone.

Does ASME B16.5 specify the correct material for every service?

No. The standard covers materials within its scope, but selecting a suitable material for a particular fluid, temperature and operating environment requires consideration of the applicable material specifications, piping code and project requirements.

Can an ASME B16.5 flange connect to an ASME B16.47 flange?

Do not assume compatibility based only on the nominal size or pressure class. Confirm the applicable standards, dimensional requirements, facing, bolt-hole arrangement and project specifications before approving the connection.

Where can I obtain the official ASME B16.5 requirements?

Consult the official ASME publication for the edition specified by your project. A general online guide can help explain the terminology, but it should not replace the standard’s complete requirements or the approved engineering documents.

Final Considerations

ASME B16.5 provides a common technical framework for specifying pipe flanges and flanged fittings, but correct selection requires more than identifying a standard number.

Engineers and buyers should verify the nominal size, pressure class, material, flange type, facing, pressure-temperature rating and applicable project requirements together. For large-diameter flanges or non-standard project conditions, confirm the governing standard and approved drawings before finalizing the specification.

For further technical information, review our ASME B16.5 flange dimensions and selection guide and our ASME B16.47 vs. B16.5 comparison.

If you are sourcing flanges for an industrial piping project, provide the required size, class, material, facing, quantity and delivery destination when contacting JS FITTINGS. These details help establish the scope of your enquiry and identify the information needed for a quotation.

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