ANSI Flange Dimensions Chart: Sizes, Classes & Standards

ANSI flanges are widely used components for connecting pipes, valves, pumps, pressure vessels, and other equipment in industrial piping systems. They create a detachable joint that can be assembled using bolts, nuts, and a gasket. Unlike welded connections, a flanged joint can be opened for inspection, cleaning, maintenance, or equipment replacement.

The term “ANSI flange” remains common in the piping industry, although most dimensional and pressure-temperature requirements are now published by the American Society of Mechanical Engineers (ASME). Therefore, products described as ANSI flanges are generally manufactured according to standards such as ASME B16.5 or ASME B16.47.

Flange selection involves more than choosing the nominal pipe size. Engineers and buyers must also consider:

• Flange standard
• Nominal Pipe Size (NPS)
• Pressure class
• Flange type
• Facing type
• Material specification
• Pipe wall thickness or schedule
• Operating pressure and temperature
• Gasket and bolting requirements
• Corrosion allowance and service conditions

An ANSI flange dimensions chart normally provides essential measurements such as flange outside diameter, flange thickness, bolt-circle diameter, number of bolt holes, bolt-hole diameter, bore diameter, and raised-face diameter. Depending on the flange type, it may also include hub dimensions, welding-neck length, socket depth, and approximate weight.

Two primary ASME standards are used for conventional pipe flange dimensions:

• ASME B16.5 generally covers flanges from NPS ½ through NPS 24.
• ASME B16.47 covers large-diameter steel flanges from NPS 26 through NPS 60.

Using the correct dimensional chart helps ensure that mating flanges have compatible bolt patterns, facing dimensions, gaskets, and pressure ratings. However, dimensional compatibility alone does not guarantee that two flanges are suitable for the same service. Material group, pressure class, temperature, facing, gasket, bolting, and applicable piping code must also be verified.

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1. What Are ANSI Flanges?

ANSI flanges are standardized mechanical components used to connect sections of pipe or attach piping to valves, pumps, vessels, and other process equipment. The expression “ANSI flange” is an established industry term, but the dimensional requirements commonly associated with it are now primarily maintained under ASME standards.

A typical flanged connection consists of:

1. Two compatible flanges
2. A gasket positioned between the flange faces
3. Stud bolts or machine bolts
4. Nuts and washers where applicable
5. A controlled tightening procedure

When the bolts are tightened, the flange faces compress the gasket. This compression allows the gasket to fill small surface irregularities and create a pressure-containing seal.

Main Functions of an ANSI Flange

ANSI flanges perform several important functions in a piping system:

• Connect two sections of pipe
• Connect pipes to valves and pumps
• Provide access for inspection and maintenance
• Allow equipment to be removed without cutting the pipe
• Close the end of a piping system
• Accommodate instruments, orifice plates, and specialty components
• Create standardized connection points between different equipment packages

Because their dimensions are standardized, flanges produced by different manufacturers can usually be connected when they comply with the same standard, size, pressure class, and facing requirements.

Common ANSI Flange Types

The most common flange types include:

Weld Neck Flange

A weld neck flange has a long tapered hub and is joined to the pipe using a full-penetration butt weld. The gradual transition between the pipe and flange helps distribute mechanical stress.

Weld neck flanges are commonly used for:

• High-pressure systems
• High-temperature services
• Cyclic operating conditions
• Hazardous fluids
• Applications with significant vibration or bending loads

The flange bore should generally match the inside diameter of the connected pipe. Therefore, the pipe schedule or wall thickness must be specified when ordering a weld neck flange.

Slip-On Flange

A slip-on flange slides over the outside of the pipe and is normally secured using fillet welds at the inside and outside of the connection.

Slip-on flanges are relatively easy to align and install. They are commonly used in lower-pressure applications where severe temperature cycling or mechanical loading is not expected.

Socket Weld Flange

A socket weld flange contains a recessed socket into which the pipe is inserted. The pipe is then joined to the flange using an external fillet weld.

This type is often used for small-bore, high-pressure piping. The required installation gap between the pipe end and socket bottom must be considered to reduce stress caused by thermal expansion during welding and operation.

Threaded Flange

A threaded flange has an internal pipe thread and can be installed without welding. It is useful where welding is restricted or where the piping must be assembled and removed easily.

Threaded flanges are generally limited to suitable pressure, temperature, fluid, and pipe-size conditions. They may not be appropriate for services involving severe vibration, temperature cycling, or crevice corrosion.

Blind Flange

A blind flange does not have a center bore. It is used to close:

• Pipe ends
• Valve outlets
• Vessel nozzles
• Manways
• Future piping connections

Because a blind flange is subjected to significant bending stress from internal pressure, its thickness is often greater than that of some other flange types of the same nominal size and pressure class.

Lap Joint Flange

A lap joint flange is used with a separate stub end. The flange can rotate around the pipe, making bolt-hole alignment easier during installation.

Because the flange itself does not normally contact the process fluid, a less expensive flange material may sometimes be combined with a corrosion-resistant stub end. Material selection must still comply with the applicable design code and project specification.

ANSI Flange Pressure Classes

ANSI/ASME flanges are grouped into pressure classes. Common classes include:

• Class 150
• Class 300
• Class 400
• Class 600
• Class 900
• Class 1500
• Class 2500

The class number is a pressure-temperature rating designation. It should not be interpreted as the flange’s allowable pressure in psi under every operating condition.

For example, a Class 150 flange does not simply have a universal maximum working pressure of 150 psi. Its allowable pressure depends on:

• Flange material
• Material group
• Operating temperature
• Applicable pressure-temperature rating table
• Piping design code
• Service conditions

As temperature increases, the allowable working pressure of a flange material generally decreases. Engineers must therefore evaluate pressure and temperature together rather than selecting a flange based only on its class designation.

Flange Facing Types

The flange facing provides the contact surface for the gasket. Common facing types include:

• Raised face
• Flat face
• Ring-type joint
• Male-and-female facing
• Tongue-and-groove facing

Raised-face flanges are widely used in process piping. Flat-face flanges are often used with cast-iron or other equipment where excessive bending caused by flange rotation could damage the mating component. Ring-type joint flanges are commonly selected for high-pressure and high-temperature services.

Both mating flanges must have compatible facing arrangements. The gasket type, gasket dimensions, surface finish, and bolting procedure must also suit the selected facing.

Common Flange Materials

ANSI/ASME flanges are manufactured from various materials, including:

• Carbon steel
• Low-temperature carbon steel
• Stainless steel
• Alloy steel
• Duplex and super duplex stainless steel
• Nickel alloys
• Copper alloys
• Cast or ductile iron

Common ASTM flange material specifications include ASTM A105 for forged carbon steel, ASTM A182 for forged alloy and stainless steel, and ASTM A350 for low-temperature forged carbon and alloy steel.

The correct material depends on operating temperature, pressure, corrosion conditions, mechanical loads, fluid compatibility, and the applicable design code.

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2. ANSI Flange Standards and Size Ranges

Different standards apply to different flange sizes, pressure classes, materials, and applications. For general industrial steel piping, ASME B16.5 and ASME B16.47 are the two principal dimensional standards.

ASME B16.5 Pipe Flanges and Flanged Fittings

ASME B16.5 covers pipe flanges and flanged fittings in nominal sizes from NPS ½ through NPS 24. Depending on size, material, and product type, it includes the following pressure classes:

• Class 150
• Class 300
• Class 400
• Class 600
• Class 900
• Class 1500
• Class 2500

The standard addresses requirements such as:

• Pressure-temperature ratings
• Materials
• Dimensions
• Tolerances
• Marking
• Testing
• Flange facings
• Bolting
• Gaskets and flange joints

ASME B16.5 dimensional tables provide values including flange outside diameter, minimum thickness, hub dimensions, bolt-circle diameter, bolt-hole diameter, number of bolt holes, and facing dimensions.

Not every flange type is available in every size and pressure class. Before using a dimension from a general chart, the engineer should confirm that the particular flange type, material, size, and class are included in the governing standard.

ASME B16.47 Large-Diameter Steel Flanges

ASME B16.47 covers large-diameter steel flanges from NPS 26 through NPS 60. The standard includes:

• Class 75
• Class 150
• Class 300
• Class 400
• Class 600
• Class 900

ASME B16.47 contains two dimensional systems: Series A and Series B.

ASME B16.47 Series A

Series A flanges are generally thicker and heavier and use larger or fewer bolts than comparable Series B flanges. They are commonly associated with demanding pipeline and industrial applications.

Because Series A flanges contain more metal, they may offer greater resistance to external loads. However, they are also heavier and can require more installation space.

ASME B16.47 Series B

Series B flanges are generally more compact and lighter than Series A flanges of the same nominal size and pressure class. Their bolt patterns normally use a larger number of smaller-diameter bolts.

Series B flanges may be selected when lower weight, reduced outside diameter, or a more compact arrangement is beneficial. Their suitability must still be confirmed through the applicable piping code, design calculations, project specifications, and equipment loads.

Series A and Series B flanges of the same NPS and pressure class are not normally interchangeable. Their outside diameters, bolt-circle diameters, bolt-hole quantities, and other dimensions can differ.

ASME B16.5 and ASME B16.47 Size Range

ASME B16.5 ends at NPS 24, while ASME B16.47 begins at NPS 26. Therefore, the applicable standard is normally determined first by nominal flange size.

ASME B16.36 Orifice Flanges

ASME B16.36 covers orifice flanges used with differential-pressure flow measurement systems. These flanges include pressure-tapping provisions on each side of an orifice plate.

Orifice flange assemblies can include:

• Orifice flanges
• Orifice plate
• Gaskets
• Jack screws
• Pressure taps
• Stud bolts and nuts

Although many general dimensions are related to ASME B16.5 flange dimensions, the pressure taps and other special features must comply with ASME B16.36.

ASME B16.48 Line Blanks

ASME B16.48 covers line blanks used to isolate sections of piping. Typical components include:

• Spectacle blinds
• Paddle blinds
• Paddle spacers

These components are installed between flanges and must match the applicable flange size, pressure class, facing, bolting arrangement, and internal bore requirements.

Other Related Flange Standards

Other standards may apply depending on the equipment and industry:

• API 6A for wellhead and Christmas-tree equipment
• API 6B and API 6BX flange connections
• MSS SP-44 for steel pipeline flanges
• AWWA flange standards for water-service applications
• EN 1092-1 for European steel flanges
• JIS B 2220 for Japanese pipe flanges

Flanges manufactured to different standards should not be assumed to be interchangeable. Even when their nominal size and pressure designation appear similar, they may have different:

• Outside diameters
• Bolt-circle diameters
• Bolt-hole quantities
• Bolt-hole diameters
• Flange thicknesses
• Facing dimensions
• Pressure-temperature ratings

NPS and DN Flange Size Designations

ANSI/ASME flange sizes are usually specified using Nominal Pipe Size. Metric projects may also show the corresponding nominal diameter designation.

Examples include:

NPS and DN are nominal designations. They do not directly represent the exact pipe outside diameter or flange bore. For example, an NPS 4 pipe has a standardized outside diameter of 4.500 inches, rather than exactly 4 inches.

The flange bore may also vary according to flange type and pipe wall thickness. This is especially important for weld neck and socket weld flanges, which must match the connected pipe dimensions.

Before selecting or ordering a flange, the complete description should include, at minimum:

• Applicable standard
• Flange type
• Nominal pipe size
• Pressure class
• Material grade
• Facing type
• Pipe schedule or bore where required
• Special coating or corrosion requirements
• Inspection and certification requirements

3. ANSI Flange Pressure Classes

ANSI/ASME flanges are divided into pressure classes that indicate their pressure-temperature capabilities. The most common flange classes specified by ASME B16.5 are:

• Class 150
• Class 300
• Class 400
• Class 600
• Class 900
• Class 1500
• Class 2500

ASME B16.47 also includes Class 75 for certain large-diameter flanges.

The pressure class affects several flange dimensions, including:

• Outside diameter
• Flange thickness
• Hub dimensions
• Bolt-circle diameter
• Number of bolt holes
• Bolt-hole diameter
• Stud-bolt diameter
• Raised-face or ring-joint dimensions

As the pressure class increases, the flange generally becomes thicker and heavier. Higher-pressure flanges may also require larger bolts or a greater number of bolts to maintain sufficient gasket compression under operating pressure.

Flange Class Is Not a Direct Pressure Rating

A common mistake is to assume that a Class 150 flange has a maximum working pressure of 150 psi. The class number is a designation rather than a universal allowable pressure.

The actual pressure rating depends on:

• Flange material
• Operating temperature
• Material group
• Applicable ASME pressure-temperature table
• Service fluid
• Design code
• Corrosion and mechanical loading

For example, a carbon-steel Class 150 flange may have an allowable pressure higher than 150 psi at ambient temperature. However, its allowable pressure decreases as the operating temperature increases.

Therefore, the flange class must be selected using the applicable ASME pressure-temperature rating table rather than the class number alone.

Class 150 Flanges

Class 150 is one of the most commonly used pressure classes in general industrial piping. It is frequently found in:

• Cooling-water systems
• Utility piping
• Low-pressure steam systems
• Compressed-air systems
• Water-treatment plants
• Low-pressure process lines

Class 150 flanges normally have thinner bodies and smaller bolting than higher-class flanges of the same nominal size.

Class 300 Flanges

Class 300 flanges are designed for higher pressure-temperature combinations than Class 150 flanges of the same material.

Compared with Class 150, a Class 300 flange generally has:

• Greater flange thickness
• A different bolt-circle diameter
• Larger or more numerous bolts
• Increased weight
• Higher pressure-temperature capability

Class 150 and Class 300 flanges of the same NPS are normally not directly interchangeable because their bolting dimensions differ.

Class 400 and Class 600 Flanges

Class 400 flanges are less commonly specified than Class 300 or Class 600. They may be found in particular refinery, power-generation, and process applications.

Class 600 flanges are widely used for higher-pressure services, including:

• High-pressure steam
• Oil and gas processing
• Chemical injection
• High-pressure process piping
• Refinery systems
• Compressor and pump connections

The increased thickness and heavier bolting help the joint withstand higher internal pressure and gasket-separation forces.

Class 900 Flanges

Class 900 flanges are intended for demanding high-pressure and high-temperature applications. They are commonly associated with:

• Refinery process lines
• High-pressure gas systems
• Petrochemical plants
• Power-generation facilities
• Offshore production systems

Class 900 flanges are significantly heavier than Class 150 or Class 300 flanges of the same nominal size.

Class 1500 and Class 2500 Flanges

Class 1500 and Class 2500 flanges are used in extremely high-pressure services. Typical applications include:

• High-pressure gas injection
• Chemical processing
• Well-service systems
• High-pressure steam
• Offshore and subsea facilities
• Severe refinery services

These flanges have substantial body thicknesses and heavy bolting arrangements. Not every flange type or nominal size is available in these pressure classes, so the applicable standard tables must be checked carefully.

Flange Class Selection

A flange class should be selected by considering the complete design conditions:

1. Determine the maximum design pressure.
2. Determine the maximum and minimum design temperatures.
3. Select the flange material specification and grade.
4. Identify the corresponding ASME material group.
5. Check the pressure-temperature rating table.
6. Consider external piping loads and thermal expansion.
7. Verify flange, gasket, and bolting compatibility.
8. Apply any additional requirements from the piping code or project specification.

The pressure class applies to the complete flanged joint. Using a high-class flange with unsuitable bolts, gasket materials, or mating equipment does not produce a properly rated connection.

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4. ANSI Flange Dimensions Explained

An ANSI flange dimensions chart uses letters or symbols to identify the critical measurements of a flange. The exact symbols may vary between manufacturers, but the principal dimensions remain similar.

Understanding these measurements is essential when selecting mating flanges, checking equipment nozzles, preparing piping drawings, or ordering replacement components.

Nominal Pipe Size

Nominal Pipe Size, abbreviated as NPS, identifies the nominal size of the pipe and flange.

Examples include:

• NPS ½
• NPS 1
• NPS 2
• NPS 4
• NPS 8
• NPS 12
• NPS 24

NPS is not necessarily equal to the measured pipe outside diameter. For example, an NPS 2 pipe has an actual outside diameter of 2.375 inches.

Flange Outside Diameter

The flange outside diameter is the maximum diameter across the flange body. It affects:

• Required installation space
• Clearance from adjacent equipment
• Flange weight
• Bolt arrangement
• Compatibility with the mating flange

Flange outside diameter normally increases with nominal size and pressure class. Two flanges with the same NPS but different pressure classes may have different outside diameters.

Flange Thickness

Flange thickness is normally measured from the back of the flange to the base of the raised face. The raised-face height is generally treated as a separate dimension.

The required thickness depends on:

• Flange type
• Nominal size
• Pressure class
• Applicable standard
• Flange material

Blind flanges may be thicker than other flange types because internal pressure subjects the solid flange plate to bending stress.

Bore Diameter

The bore is the central opening through the flange. Its required diameter depends on the flange type and connected pipe.

For weld neck flanges, the bore should match the inside diameter of the selected pipe schedule as closely as practical. Therefore, an order for a weld neck flange should normally include both NPS and pipe schedule.

Slip-on, threaded, socket weld, and lap joint flanges have different bore configurations and should be checked against the applicable dimensional table.

Bolt-Circle Diameter

The bolt-circle diameter, sometimes called the pitch circle diameter or PCD, is the diameter of the imaginary circle passing through the centers of all bolt holes.

It is one of the most important dimensions for determining whether two flanges can be bolted together.

The bolt-circle diameter must not be confused with:

• Flange outside diameter
• Raised-face diameter
• Bore diameter
• Bolt-hole diameter

Two flanges will not align if their bolt circles are different, even if they have the same nominal pipe size.

Number of Bolt Holes

The number of bolt holes depends on flange size and pressure class. Smaller flanges may have four bolt holes, while large or high-pressure flanges may have considerably more.

Bolt holes are equally spaced around the bolt circle. For conventional piping installation, flanges are commonly positioned so that bolt holes straddle the vertical and horizontal centerlines rather than placing a bolt hole directly on the centerline.

Bolt-Hole Diameter

Bolt holes are larger than the nominal bolt diameter to provide assembly clearance. For example, a flange designed for a ⅝-inch bolt will have a bolt hole larger than ⅝ inch.

The bolt-hole diameter should not be used to determine the required bolt size without consulting the relevant table.

Stud-Bolt Diameter and Length

Flanged joints are commonly assembled using stud bolts with a nut at each end. The required stud-bolt diameter depends on:

• Flange standard
• NPS
• Pressure class
• Number of bolt holes
• Flange type and facing

Stud-bolt length also depends on flange thickness, gasket thickness, nut dimensions, washers, and the required thread projection beyond the nuts.

Raised-Face Diameter

The raised face is the gasket-seating surface located around the flange bore. Its outside diameter is standardized according to flange size and pressure class.

The gasket must fit within the raised-face seating area without extending into the pipe bore or interfering with the bolts.

Raised-Face Height

Raised-face height is measured above the main flange face. Common dimensional practice under ASME standards uses different raised-face heights depending on pressure class.

The raised-face height is generally excluded from the listed minimum flange thickness unless otherwise stated in the applicable table or drawing.

Hub Diameter and Hub Length

Weld neck, slip-on, socket weld, and threaded flanges may include a hub. Relevant hub dimensions can include:

• Diameter at the base
• Diameter at the welding end
• Total length through the hub
• Tapered transition length

The hub transfers mechanical loads between the flange and pipe. Weld neck flanges have a particularly long tapered hub to reduce stress concentration.

Approximate Flange Weight

Manufacturer charts may include approximate flange weights. Weight is useful for:

• Material take-offs
• Transportation planning
• Lifting arrangements
• Support calculations
• Cost estimation

Published weights are normally approximate because actual weight can vary with material density, bore size, manufacturing tolerance, and pipe schedule.

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5. ANSI Flange Dimensions Chart by Pressure Class

The following tables provide commonly referenced flange dimensions in inches. They are intended for preliminary identification and comparison. Final dimensions must be verified against the applicable edition of ASME B16.5 or ASME B16.47 and the specified flange type.

Dimensions such as thickness, bore, and hub length are not included in the general tables because they can vary by flange type and pipe schedule.

Class 150 Flange Dimensions Chart

Class 150 flanges are commonly used in low- and moderate-pressure industrial services. Their relatively compact dimensions and lower weight make them economical when the pressure-temperature rating is sufficient for the application.

Class 300 Flange Dimensions Chart

Class 300 flanges generally have larger outside diameters, thicker bodies, and heavier bolting arrangements than Class 150 flanges. Their bolt patterns are different, so the two classes cannot normally be connected directly.

Class 400 and Class 600 Flanges

Class 400 and Class 600 flange dimensions are similar for many nominal sizes, but their pressure-temperature ratings and minimum thickness requirements must be checked separately.

These pressure classes generally require:

• Thicker flange sections
• Larger bolts
• Increased bolt preload
• Stronger gasket arrangements
• More installation clearance
• Heavier lifting and supporting provisions

They are commonly used in refinery, chemical-processing, power-generation, and high-pressure utility systems.

Class 900 Flanges

Class 900 flanges are considerably heavier than lower-class flanges. They require careful evaluation of:

• Equipment-nozzle loads
• Pipe support locations
• Flange alignment
• Bolt-tightening method
• Gasket selection
• Installation clearance

For some smaller sizes, the flange outside diameter may be shared with another high-pressure class, but flange thickness, bore, hub, and pressure-temperature rating may differ. Identical outside dimensions do not mean that the flanges have identical ratings.

Class 1500 and Class 2500 Flanges

Class 1500 and Class 2500 flanges are designed for severe pressure-temperature conditions. Their large thickness and bolting requirements can significantly affect piping layout and equipment design.

Special attention should be given to:

• Flange and fitting availability
• Pipe wall thickness
• Weld-end preparation
• Required bolt tension
• Gasket type
• Material strength
• Thermal expansion
• Fatigue and cyclic loading

For these pressure classes, flange selection should be based directly on the governing standard and project specification rather than on a simplified general-purpose chart.

Important Notes for Using the Charts

Before applying any flange dimension, confirm all of the following:

• The correct flange standard
• Nominal pipe size
• Pressure class
• Flange type
• Facing type
• Material specification
• Pipe schedule or bore
• Units of measurement
• Applicable standard edition
• Dimensional tolerances

A flange should never be ordered using only its nominal size. A complete description might read:

ASME B16.5, NPS 4, Class 300, weld neck flange, raised face, ASTM A105, bore for Schedule 80 pipe.