Flange Size & Dimension Guide

Understanding flange dimensions is important when selecting, replacing or manufacturing a flange for an industrial piping system. A flange is defined by more than its nominal pipe size. Outside diameter, thickness, bore, bolt circle, bolt-hole size, number of holes, facing and flange type all need to match the applicable specification.

Vandan Alloys supplies industrial flanges in different sizes, pressure classes, materials and configurations according to applicable standards and customer requirements.

Understanding Flange Size

Flange size is commonly identified using NPS (Nominal Pipe Size) in ASME-based piping systems or DN (Nominal Diameter) in many European specifications.

The nominal size does not represent the actual outside diameter of the flange.

For example, an NPS designation identifies the nominal pipe size, while the actual flange dimensions are established by the applicable standard, pressure class and flange type.

NPS Flange Dimensions

ASME-based flange specifications commonly use NPS sizes.

For standard pipe flanges covered by ASME B16.5, the dimensional range is NPS 1/2 through NPS 24.

The actual dimensions depend on factors such as:

NPS Size, Pressure Class, Flange Type, Facing and Material Specification.

Therefore, an NPS number alone is not sufficient to determine every flange dimension.

DN Flange Dimensions

European flange systems commonly use DN (Nominal Diameter) designations.

DN sizes are used in standards such as EN 1092-1, where dimensions and pressure designations are defined according to the applicable flange type and specification.

When ordering a DN flange, the DN size, PN rating, flange type, facing and standard should be clearly specified.

Main Flange Dimensions

The most important flange dimensions normally considered during identification or inspection include:

Flange Outside Diameter (OD) – The overall outside diameter of the flange.

Flange Thickness – The thickness of the main flange body as defined by the applicable standard.

Bore / Inside Diameter – The internal opening through which the pipe or equipment connection passes.

Bolt Circle Diameter (PCD) – The diameter of the circle passing through the centers of the flange bolt holes.

Number of Bolt Holes – The quantity of holes provided for bolting the flange connection.

Bolt Hole Diameter – The diameter of each bolt hole.

Raised Face Diameter – Applicable where the flange has a raised-face configuration.

Hub Dimensions – Important for weld neck and other flanged configurations with hubs.

Flange Outside Diameter

The outside diameter (OD) is measured across the widest circular portion of the flange body.

The flange OD changes according to the nominal size, pressure class and applicable standard.

It should not be selected based only on the pipe's outside diameter.

Flange Thickness

Flange thickness is an important structural dimension.

The required thickness depends on the flange standard, nominal size, pressure class, material and flange configuration.

For standard flanges, the thickness should be taken from the applicable dimensional table rather than estimated.

When a customer specifies a flange drawing, the drawing dimensions should be checked against the applicable standard before manufacturing.

Flange Bore / Inside Diameter

The bore is the internal opening of the flange.

For many flange requirements, the bore is related to the connected pipe dimensions and applicable flange specification.

The exact bore should be confirmed when ordering, especially for:

Weld Neck Flanges, special bore requirements, equipment connections and custom flanges.

Flange PCD / Bolt Circle Diameter

PCD (Pitch Circle Diameter) or bolt circle diameter is the diameter on which the centers of the bolt holes are located.

PCD is critical for matching two mating flanges.

Even when the nominal size appears identical, different standards or pressure classes can have different bolt-circle arrangements.

Number of Bolt Holes

The number of bolt holes is determined by the applicable flange standard, size and pressure class.

The bolt-hole arrangement must match the mating flange.

Important details include:

Number of Holes, Hole Diameter, Bolt Circle Diameter and Bolt Size.

Flange Bolt Hole Diameter

Bolt-hole diameter is specified according to the applicable flange standard and bolting arrangement.

The bolt-hole size should not be selected simply from the bolt nominal diameter without checking the relevant standard.

Correct hole diameter and bolt arrangement are essential for proper assembly.

Weld Neck Flange Dimensions

A Weld Neck Flange includes a hub and tapered transition designed for butt-welding to the pipe.

Important dimensions can include:

Outside Diameter, Thickness, Bore, Hub Diameter, Hub Length, Bolt Circle Diameter, Number of Bolt Holes and Facing Dimensions.

The weld-neck bore and welding-end configuration should be confirmed according to the pipe schedule or specified requirement.

Blind Flange Dimensions

A Blind Flange is used to close a piping connection.

Typical dimensional requirements include:

Outside Diameter, Thickness, Bolt Circle Diameter, Number of Bolt Holes, Bolt Hole Diameter and Facing Dimensions.

Blind flange dimensions vary according to the applicable standard and pressure class.

Slip-On Flange Dimensions

A Slip-On Flange is installed over the pipe and welded in position.

Its dimensional requirements may include:

Outside Diameter, Thickness, Bore, Bolt Circle Diameter, Number of Bolt Holes and Facing Dimensions.

The bore should be suitable for the specified pipe and flange configuration.

Socket Weld Flange Dimensions

A Socket Weld Flange is commonly used for smaller-diameter piping applications.

In addition to the normal flange dimensions, the socket configuration and bore details are important.

The socket dimensions must be compatible with the specified pipe size and applicable standard.

Threaded Flange Dimensions

A Threaded Flange contains an internal thread for connection to externally threaded pipe.

Important details include:

Flange OD, Thickness, PCD, Bolt Holes, Facing and Internal Thread Specification.

Thread requirements should be clearly specified because thread standards can vary.

Lap Joint Flange Dimensions

A Lap Joint Flange is generally used with a compatible stub end.

The flange dimensions include the normal bolting and facing details, while the stub-end dimensions must also match the piping requirement.

RTJ Flange Dimensions

Ring Type Joint (RTJ) Flanges require precise machining of the ring-groove area.

Important dimensions can include:

RTJ Groove Diameter, Groove Width, Groove Depth, Flange Thickness, Bolt Circle and Bolt-Hole Arrangement, according to the applicable standard.

RTJ dimensions should always be taken from the relevant standard or approved drawing.

ASME B16.5 Flange Dimensions

ASME B16.5 covers pipe flanges and flanged fittings for NPS 1/2 through NPS 24 and establishes dimensional and related requirements for applicable flange products.

The required dimensional data depends on:

NPS, Class, Flange Type and Facing.

For an accurate flange dimension, the complete designation should therefore be specified rather than only the pipe size.

ASME B16.47 Large Diameter Flange Dimensions

For large-diameter steel flanges, ASME B16.47 covers sizes NPS 26 through NPS 60. The current 2025 edition includes pressure-temperature ratings, materials, dimensions, tolerances, marking and testing requirements.

ASME B16.47 also includes different flange series, so the Series A or Series B requirement should be confirmed for applicable orders.

Flange Pressure Class & Dimensions

Flange dimensions are closely associated with the pressure class.

Common requirements include:

Class 150, Class 300, Class 600, Class 900, Class 1500 and Class 2500, where applicable to the relevant standard and size.

A change in pressure class can change the flange thickness, OD, bolt arrangement and other dimensions.

Therefore, a flange should always be identified by its complete specification.

Flange Facing Dimensions

The flange facing must match the mating component and gasket arrangement.

Common configurations include:

Raised Face (RF), Flat Face (FF) and Ring Type Joint (RTJ).

Facing dimensions are controlled by the applicable standard and can vary according to flange size, type and pressure class.

How to Read a Flange Drawing

A flange drawing normally identifies the important dimensional and specification information required for manufacturing or inspection.

Check the drawing for:

Flange Type → Size → Class/PN Rating → Material → Standard → OD → Thickness → Bore → PCD → Bolt Holes → Facing → Hub Dimensions.

For special flanges, the drawing may also specify machining tolerances, surface finish, weld preparation or other technical requirements.

Standard Flange Dimensions vs Custom Dimensions

Standard flanges are manufactured according to recognized dimensional standards.

Custom flanges may be produced according to an approved customer drawing or project specification, subject to manufacturing feasibility and material requirements.

Custom requirements may involve:

Special Bore, Modified PCD, Non-Standard Thickness, Special Facing, Special Hub Dimensions or Custom Drilling.

A drawing should be reviewed before confirming a non-standard flange.

Flange Dimension Inspection

During dimensional inspection, the required dimensions can be checked against the approved drawing or applicable standard.

Inspection may include:

OD, Thickness, Bore, PCD, Bolt-Hole Diameter, Number of Holes, Facing, Hub Dimensions and Overall Dimensions.

Dimensional inspection requirements can be agreed according to the purchase order or inspection specification.

Flange Size & Dimension Supplier from India

Vandan Alloys supplies industrial flanges according to recognized standards and customer specifications.

We support requirements for standard flange dimensions, large-diameter flanges, drawing-based flanges and special dimensional requirements.

For an accurate quotation, provide the complete flange details including:

Type, Size, Material, Class/PN Rating, Facing, Standard, Quantity and Delivery Location.

For special dimensions, please share the technical drawing or dimensional specification.

Request a Flange Dimension-Based Quotation

Send your flange drawing or complete dimensional requirement to Vandan Alloys for technical review.

Vandan Alloys 

Manufacturer of All Types of Flanges

3/5, Sakar Bhavan, Ground Floor, 6th  Khetwadi lane, SVP Road ,Mumbai – 400 004. India

TEL +91 022 6610 9925 Mobile no: 0 9892882255  GSTIN:  27AUMPS2067F1ZB

Email: info@vandanalloys.com ; Web: www.vandanalloys.com

Please visit Our Flanges Division:www.vandanalloys.net

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Complete flange size and dimension guide covering NPS, DN, OD, thickness, bore, PCD, bolt holes, facing and ASME B16.5 & B16.47 requirements.

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