Pipe Flange Size Chart: ASME, ANSI and EN Dimensions

Pipe flanges are essential components used to connect pipes, valves, pumps, pressure vessels, and other equipment in industrial piping systems. Unlike welded connections, a flanged joint can be assembled and disassembled, making inspection, cleaning, maintenance, and equipment replacement much easier.

However, selecting a flange involves more than matching its nominal pipe size. Flange dimensions vary according to pressure class, flange type, applicable standard, material, and facing configuration. Important measurements include the flange outside diameter, thickness, bolt circle diameter, number and diameter of bolt holes, bore diameter, and raised-face dimensions.

This pipe flange size chart guide explains the dimensions and terminology used for common ASME and EN flanges. It also helps you compare flange standards, pressure ratings, and nominal sizes so that you can identify an existing flange or select the correct flange for a new piping system.

1. What Is a Pipe Flange?

A pipe flange is a circular or square mechanical component used to create a detachable connection between pipes or between a pipe and a piece of equipment. A typical flanged joint consists of two matching flanges, a gasket placed between the flange faces, and a set of bolts or studs that clamp the assembly together.

When the bolts are tightened, they apply compressive force to the gasket. The compressed gasket fills small surface irregularities between the two flange faces and creates a pressure-tight seal. The sealing performance of the connection depends on the compatibility of the flanges, gasket, bolting materials, operating conditions, and installation procedure.

Pipe flanges are commonly manufactured from:

• Carbon steel
• Stainless steel
• Alloy steel
• Duplex and super duplex stainless steel
• Cast iron and ductile iron
• Nickel alloys
• Copper alloys
• PVC, CPVC, and other plastics

The selected material must be compatible with the process fluid, operating temperature, design pressure, external environment, and connected piping material.

Common Pipe Flange Types

Pipe flanges are available in several designs, each intended for particular installation conditions:

• Weld neck flange: Butt-welded to the pipe through a tapered hub and commonly used for high-pressure, high-temperature, or cyclic service.
• Slip-on flange: Slides over the pipe and is normally secured using internal and external fillet welds.
• Socket weld flange: The pipe is inserted into a recessed socket and joined with an external fillet weld.
• Threaded flange: Connects to a pipe using internal threads and does not normally require welding.
• Lap joint flange: Used with a stub end, allowing the backing flange to rotate for easier bolt-hole alignment.
• Blind flange: Has no center bore and is used to close the end of a pipe, valve, vessel nozzle, or flanged opening.
• Orifice flange: Includes pressure-tapping connections and is used with an orifice plate for flow measurement.

How Pipe Flange Size Is Defined

A flange is primarily identified by its nominal pipe size, pressure class, flange type, facing type, and material specification. For example:

NPS 4 Class 300 RF Weld Neck Flange, ASTM A105, ASME B16.5

This designation indicates:

• NPS 4: Nominal pipe size of 4 inches
• Class 300: ASME pressure–temperature rating class
• RF: Raised-face sealing surface
• Weld neck: Flange design
• ASTM A105: Forged carbon-steel material
• ASME B16.5: Applicable dimensional and rating standard

The nominal size does not represent the actual outside diameter of the flange. For example, an NPS 4 Class 150 flange has an outside diameter considerably larger than 4 inches. Therefore, an unknown flange should not be identified using its outside diameter alone.

Main Functions of Pipe Flanges

Pipe flanges perform several important functions in piping systems:

• Connect separate pipe sections
• Connect piping to valves, pumps, compressors, and vessels
• Provide access for inspection and maintenance
• Allow equipment to be removed without cutting the pipe
• Close unused piping or equipment openings
• Support changes in piping materials or connection methods
• Provide measuring points for flow instruments and other devices

For a safe connection, the two mating flanges must have compatible nominal sizes, dimensional standards, pressure classes, facing arrangements, and bolt patterns. Flanges with the same nominal pipe size may still be incompatible if they are manufactured to different standards or pressure classes.

2. Pipe Flange Terminology and Key Dimensions

Understanding flange terminology is essential when reading a pipe flange size chart. A flange cannot be correctly identified using only its outside diameter because several dimensional and design characteristics must be considered together.

Nominal Pipe Size

Nominal Pipe Size (NPS) is the North American designation used to identify the nominal size of pipes, fittings, and flanges. It is not normally equal to the actual pipe outside diameter or the flange outside diameter.

For metric piping systems, flange size is commonly designated by Nominal Diameter (DN). Typical approximate equivalents include:

NPS-to-DN conversion is nominal rather than mathematical. For example, multiplying every NPS value by 25.4 will not necessarily produce its standardized DN designation.

Flange Outside Diameter

The outside diameter, often represented by OD or O, is measured across the widest part of the flange. It affects flange strength, bolt placement, installation clearance, and compatibility with nearby equipment.

Flanges with the same NPS but different pressure classes generally have different outside diameters. In many sizes, a higher-pressure flange is larger and thicker to accommodate larger or additional bolts.

Flange Thickness

Flange thickness is the distance between the back of the flange and the flange face, excluding certain raised-face dimensions depending on the applicable standard. It may be represented by T, C, or another symbol in manufacturers’ dimensional tables.

Higher-pressure classes generally require thicker flanges to withstand greater internal pressure and bolt loading. However, the exact thickness depends on the flange size, type, class, material, and standard.

Bolt Circle Diameter

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

It is one of the most useful dimensions for identifying an unknown flange. Two flanges can be bolted together only when their bolt-circle diameters, bolt-hole quantities, and hole sizes are compatible.

Bolt-Hole Diameter and Quantity

A flange size chart specifies both the number of bolt holes and the diameter of each hole. Bolt holes are slightly larger than the corresponding bolts or studs to provide assembly clearance.

As the nominal size or pressure class increases, the flange may require:

• More bolts or studs
• Larger bolt diameters
• A larger bolt circle
• Greater flange thickness

Bolt-hole diameter should not be confused with bolt diameter. The required stud or bolt size is normally smaller than the flange hole.

Flange Bore

The bore is the central opening through which the process fluid passes. Its dimensions depend on the flange type and intended pipe connection.

For weld neck and socket weld flanges, the bore may need to match the pipe schedule. A mismatch can create an internal step that increases turbulence, erosion, pressure loss, or contamination retention.

Slip-on, threaded, lap joint, and blind flanges have different bore requirements. A blind flange has no flow bore because it is designed to close a flanged opening.

Hub Diameter and Length

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

• Diameter at the base of the hub
• Diameter at the welding end
• Overall length through the hub
• Hub taper

The tapered hub of a weld neck flange gradually transfers stress from the flange to the pipe, making it suitable for severe, cyclic, or high-temperature service.

Raised-Face Diameter and Height

A raised-face flange has an elevated circular sealing area around the bore. The gasket is positioned on this surface rather than covering the entire flange face.

Under ASME B16.5, the standard raised-face height is generally:

• 1.6 mm (1/16 in.) for Classes 150 and 300
• 6.4 mm (1/4 in.) for Classes 400 through 2500

The raised-face height may or may not be included in the listed flange thickness, depending on the dimension and note used in the applicable standard.

Flange Facing

The flange facing is the surface that contacts and compresses the gasket. Common facing arrangements include:

• Raised Face (RF)
• Flat Face (FF)
• Ring-Type Joint (RTJ)
• Tongue and Groove (T&G)
• Male and Female (M&F)

The selected gasket must be compatible with the flange facing, pressure class, process fluid, and operating temperature.

3. Pipe Flange Types and Their Dimensions

Different flange types may share the same outside diameter and bolt pattern when manufactured to the same size, pressure class, and standard. However, their hubs, bores, thicknesses, attachment methods, and overall lengths can differ substantially.

Weld Neck Flange Dimensions

A weld neck flange has a long, tapered hub and a butt-weld end. Its bore is normally specified to match the inside diameter of the connected pipe.

Important dimensions include:

• Flange outside diameter
• Flange thickness
• Overall length through the hub
• Hub diameter
• Welding-end diameter
• Bore diameter
• Bolt circle and bolt holes

Because the flange bore depends on the connected pipe wall thickness, weld neck flanges must normally be ordered for a specific pipe schedule, such as Schedule 40, Schedule 80, or Schedule 160.

Weld neck flanges are commonly selected for high pressure, high temperature, severe vibration, large temperature changes, and cyclic loading.

Slip-On Flange Dimensions

A slip-on flange has a bore slightly larger than the outside diameter of the pipe, allowing the flange to slide over the pipe. It is normally attached using fillet welds on both the inside and outside.

Its key dimensions include:

• Flange outside diameter
• Flange thickness
• Bore diameter
• Hub diameter and length
• Bolt circle
• Bolt-hole quantity and diameter

Slip-on flanges are easier to align than weld neck flanges, but they generally provide lower fatigue strength and are less suitable for severe cyclic service.

Socket Weld Flange Dimensions

A socket weld flange contains a recessed socket into which the pipe is inserted. A small expansion gap is left between the pipe end and the bottom of the socket before welding.

Important dimensions include:

• Flange outside diameter
• Flange thickness
• Socket bore
• Socket depth
• Internal bore
• Hub dimensions
• Bolt pattern

Socket weld flanges are commonly used with small-bore, high-pressure piping. The flange bore and socket dimensions must be compatible with both the nominal pipe size and pipe schedule.

Threaded Flange Dimensions

A threaded flange has an internally threaded bore that connects to external pipe threads. It can be installed without welding, making it useful where hot work is restricted.

The main dimensions include:

• Flange outside diameter
• Flange thickness
• Thread size and type
• Hub diameter and length
• Bolt circle
• Bolt-hole dimensions

The thread standard must match the connected pipe. ASME threaded flanges commonly use tapered NPT threads unless another thread form is specified.

Lap Joint Flange Dimensions

A lap joint flange is used with a separate stub end. The stub end is welded to the pipe, while the loose backing flange rotates behind it.

Important dimensions include:

• Flange outside diameter
• Flange thickness
• Bore diameter
• Inside corner radius
• Bolt pattern
• Stub-end lap diameter
• Stub-end length

Because the backing flange does not normally contact the process fluid, a less corrosion-resistant material may sometimes be used for it, while the wetted stub end is manufactured from a compatible alloy.

Blind Flange Dimensions

A blind flange is a solid plate without a center bore. It is used to terminate a piping run or close a valve, pressure vessel nozzle, or equipment opening.

Key dimensions include:

• Outside diameter
• Flange thickness
• Bolt circle diameter
• Number and size of bolt holes
• Facing dimensions

Blind flanges are subjected to significant bending stress because there is no central opening to relieve the pressure load. Consequently, their thickness can differ considerably from that of other flange types.

Orifice Flange Dimensions

Orifice flanges are supplied as a matched pair for holding an orifice plate in a flow-measurement installation. They typically include pressure-tapping holes and jack screws for separating the flanges during maintenance.

In addition to standard flange dimensions, an orifice flange assembly may specify:

• Pressure-tap size and location
• Jack-screw size
• Orifice plate thickness
• Gasket dimensions
• Inside diameter
• Flange separation requirements

The dimensional arrangement must match the applicable piping and flow-measurement requirements.

4. ASME B16.5 Pipe Flange Size Chart

ASME B16.5 covers pipe flanges and flanged fittings from NPS 1/2 through NPS 24. It includes pressure–temperature ratings, materials, dimensions, tolerances, marking, testing, and methods for designating flange openings.

The standard includes the following pressure classes:

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

A pressure class is not a direct statement of the flange’s allowable pressure in psi. The allowable working pressure depends on the flange material and operating temperature.

ASME B16.5 Class 150 Flange Size Chart

The table below provides common Class 150 dimensional data. Dimensions are presented in inches and millimeters.

How to Read the Chart

For an NPS 4 Class 150 flange, the chart indicates:

• Flange outside diameter: 9.00 in. or 228.6 mm
• Bolt circle diameter: 7.50 in. or 190.5 mm
• Number of bolt holes: 8
• Bolt-hole diameter: 0.75 in. or 19.1 mm
• Nominal bolt size: 5/8 in.

These bolting dimensions are generally shared by different flange types of the same NPS and pressure class. However, flange thickness, bore, hub dimensions, and overall length vary according to whether the flange is weld neck, slip-on, threaded, socket weld, lap joint, or blind.

Factors That Change ASME Flange Dimensions

Flange dimensions may change according to:

• Nominal pipe size
• Pressure class
• Flange type
• Pipe schedule
• Facing type
• Material group
• Standard edition
• Special design requirements

For procurement, fabrication, or engineering work, dimensions should always be verified using the applicable edition of ASME B16.5 and the approved manufacturer’s drawing.