Flange Dimensions Explained – Complete Guide for Piping Professionals

When working with piping systems, understanding flange dimensions is not just a technical detail — it’s the key to ensuring structural integrity, leak-proof sealing, and long-term performance. At Udhhyog, we’ve supported thousands of B2B buyers, engineers, and project consultants across India who face daily challenges in identifying, matching, or replacing flanges.

This blog demystifies flange dimensions, explains what each parameter means, how they interact, and why choosing the correct size matters. Whether you're dealing with stainless steel, GI, or MS flanges, this guide will help you make informed buying decisions and avoid costly project mistakes.

What Are Flange Dimensions?

Flange dimensions refer to the set of standardized physical measurements that define a pipe flange. These measurements include several parameters — not just one or two — that ensure a proper fit with pipes, bolts, gaskets, and adjoining flanges. When we talk about flange dimensions, we are referring to outer diameter, nominal bore, bolt circle diameter, bolt hole size, thickness, and face type.

Each of these is defined according to a global standard such as ASME B16.5, ASA, DIN, or IS. In India, most flange manufacturing and supply is based on ANSI/ASA dimensions for Classes 150, 300, 600, and above. Knowing these standards — and matching them with your system design — is essential.

Why Flange Dimensions Matter in Engineering Projects

Every pipeline setup is unique. One misaligned flange can cause significant problems — leakage, gasket failure, pipe strain, or even mechanical breakdown under pressure. Incorrect flange dimensions often result in:

• Improper bolt hole alignment
• Gasket misfit or over-compression
• Reduced sealing strength
• Incompatibility with valves or pipe ends
• System downtime and financial losses

At Udhhyog, we frequently assist buyers who are replacing older flanges or retrofitting systems, and in nearly every case, it’s the flange dimensions that determine the success of the installation — not just the nominal size.

Standard Parameters of Flange Dimensions

Let’s walk through each dimension that defines a flange:

• Outer Diameter (OD): This is the total width of the flange from one edge to the other. OD is the first thing most engineers check, but it is not enough on its own to identify the flange type.
• Nominal Bore (NB): The inner hole diameter that matches the pipe size. This is the actual ‘pipe size’ the flange fits. A flange with NB 50 is meant for a 2-inch pipe.
• Bolt Circle Diameter (BCD): The diameter of the circle formed by the centers of all the bolt holes. This is a crucial parameter to match when aligning two flanges.
• Number of Bolt Holes: Each pressure class and pipe size has a specific bolt pattern. This ensures secure clamping and leak-proof sealing.
• Bolt Hole Diameter: Refers to the width of each bolt hole. It determines the bolt size to be used.
• Flange Thickness: The thickness of the flange body, not including any raised face. It directly affects the pressure-handling capability.
• Raised Face Height (if applicable): In RF flanges, the raised face adds a few millimeters to the total thickness and affects gasket selection.

Each of these values varies depending on the pressure class. A 2-inch flange in Class 150 will have different bolt dimensions than a 2-inch flange in Class 300.

Understanding Flange Dimensions with Real Examples

Take the case of a 4-inch Class 150 slip-on flange. According to ANSI B16.5 standards, the outer diameter is approximately 229 mm, the bolt circle diameter is 190 mm, there are 8 bolt holes of 19 mm diameter, and the flange thickness is 24 mm.

Now compare it to a 4-inch Class 300 flange — the OD might remain similar, but the bolt circle changes to 200 mm, with bolt hole diameter increased to 22 mm and thickness up to 30 mm. These subtle differences are invisible to the untrained eye but can cause full-scale system failure if mismatched.

Raised Face, Flat Face & RTJ – Dimensional Considerations

Face type directly affects flange dimensions. Raised face (RF) flanges have an additional sealing surface raised by about 1.5 to 3 mm above the flange body. This is important when calculating total thickness and selecting gaskets. Flat face (FF) flanges do not have this elevation, and are typically used with cast iron or low-pressure systems.

Ring-type joint (RTJ) flanges have a groove machined into their face to accept a metal ring gasket, and their dimensions differ significantly in both thickness and face machining. Always confirm the face type before referring to standard dimensions.

Flange Standards That Define Dimensions

In India, the following standards are commonly used:

• ANSI/ASME B16.5: Widely used in process plants and water systems. Covers Class 150 to 2500 flanges.
• ASA (American Standards Association): An older term often used interchangeably with ANSI for Class 150 flanges.
• IS 1538 / IS 6392: Indian Standards for MS flanges used in structural and waterline systems.
• DIN: Used in select industries with German or EU-imported machinery.

Each standard has its own dimensional values even for the same nominal bore, so cross-reference is essential.

Use-Case: Incorrect Flange Dimensions in Fire Pump Room

A contractor in Ahmedabad once ordered 6-inch MS flanges for a fire pump room based on rough OD measurements. When the delivery reached site, the bolt holes didn’t align with the existing pump flange. It turned out the original flanges were Class 300 ASA type while the ordered ones were IS standard Class 150. The mismatch delayed commissioning by 3 days. After consulting Udhhyog and sharing the correct BCD and bolt hole specs, the client received matching flanges within 24 hours.

How to Cross-Check Flange Dimensions On-Site

If you are unsure of the flange specs but have the component in hand, use the following approach:

• Measure OD and BCD using a scale or caliper
• Count the bolt holes and measure their diameter
• Confirm the pipe NB
• Compare your measurements with a standard flange chart (ANSI or IS)
• Match pressure class based on thickness or existing system design

Udhhyog offers dimension charts in both mm and inch units for all popular flange types. These can be printed and used directly on-site.

Can Two Flanges with the Same NB Have Different Dimensions?

Yes. This is one of the most common causes of flange mismatch. A 3-inch NB flange can come in Class 150, 300, or 600 — all with different bolt circles, hole sizes, and thicknesses. Unless you confirm the pressure class, simply matching NB is not enough.

Also, slip-on and weld neck flanges have different hub profiles and body thicknesses, affecting how they fit with pipe and gasket assemblies.

Using Udhhyog’s Flange Dimension Chart

Udhhyog provides a verified flange dimension chart for ANSI B16.5 and IS 1538 flanges. This includes NB size, OD, BCD, bolt details, and flange thickness — organized by pressure class. We also offer custom dimension matching if you send us photos with measurements.

These charts are helpful for:

• Retrofit projects
• Firefighting lines
• Borewell systems
• Marine and process plant pipelines
• Custom flange manufacturing

Our team has helped EPC contractors across India use these charts for estimating quantities and finalizing procurement.

Final Thoughts on Flange Dimensions

Flange dimensions are not just numbers on paper — they are the foundation of a reliable piping joint. A few millimeters off in bolt circle or face thickness can ruin entire assembly operations. If you're ever in doubt, consult a trusted supplier, refer to a verified chart, and confirm face type and pressure class.

At Udhhyog, we work with OEMs, contractors, maintenance heads, and project engineers to ensure every flange fits right the first time.