Flange Size Chart in MM & Inch – A Complete Guide for Engineers

Understanding flange sizes is a fundamental part of piping design, engineering procurement, and on-site maintenance. Whether you're working in oil and gas, HVAC, water systems, or structural fabrication, you’ve likely come across the need to refer to a flange size chart — yet many buyers and technicians still face challenges in interpreting or using one.

At Udhhyog, we regularly help B2B customers who struggle with selecting the correct flange size for their specific application. This guide explains what flange sizes truly represent, how to read a size chart, and what parameters must be understood before placing an order.

Introduction to Flange Sizes

Flange sizes refer to the dimensions of a pipe flange — including its inner bore, outer diameter, bolt hole layout, thickness, and pressure compatibility. These are not just random values. They are standardized across the piping industry using global codes like ANSI, ASME, and IS. However, without a proper understanding of how these measurements are defined and listed in a flange size chart, engineers may end up with incorrect selections.

Flange sizes vary based on the nominal bore (NB) of the pipe, the pressure rating of the system, and the type of flange face. It’s important to note that even for the same NB size, the outer diameter and bolt circle can vary significantly depending on the pressure class.

Why Flange Size Charts Are Critical

Using a verified flange size chart eliminates confusion during procurement and assembly. It provides all the essential measurements at a glance — helping buyers, engineers, and suppliers match dimensions with real-world applications. This is especially important when:

• Retrofitting flanges on existing pipelines
• Matching gaskets with exact bolt hole patterns
• Ordering SS, GI, or MS flanges in bulk
• Ensuring compatibility between flange and valve connections

We’ve witnessed projects across Maharashtra, Gujarat, and Tamil Nadu where using the wrong flange size resulted in pressure failures, leakage, or costly replacement of entire pipe sections. A trusted flange size chart can prevent these errors.

What Is a Flange Size Chart?

A flange size chart is a tabulated or structured reference showing the dimensional data of standard flanges. It typically includes:

• Nominal Bore (NB)
• Outer Diameter (OD)
• Bolt Circle Diameter (BCD)
• Number of Bolt Holes
• Bolt Hole Diameter
• Flange Thickness
• Face Type Considerations (RF, FF)

This chart is organized either in millimeters (mm) or inches, and usually sorted by pressure classes such as Class 150, 300, or 600. At Udhhyog, we provide both mm and inch-based charts upon request in PDF format for engineers and purchase heads.

The Role of Nominal Bore and Pressure Class in Flange Sizing

The Nominal Bore (NB) is the base measurement. It refers to the inner diameter of the pipe the flange will connect to. The pressure class — like Class 150 or Class 300 — determines the flange’s thickness, bolt size, and bolt pattern.

For example, a 4-inch flange in Class 150 will have a different OD, BCD, and thickness compared to a 4-inch flange in Class 300. That’s why referencing just the NB is not sufficient — the full flange size chart must be consulted based on pressure requirements.

How to Read a Flange Size Chart in MM and Inch

Reading a flange size chart is simple once you understand what each column represents. Let’s consider a 3-inch flange:

• NB: 3 inch (or 80 mm)
• OD: Outer diameter, say 190 mm
• BCD: Bolt circle diameter, maybe 152 mm
• Bolt Holes: 4 holes
• Hole Diameter: 18 mm
• Thickness: 19 mm

These values will change if you move from Class 150 to Class 300. This is why Udhhyog recommends using a verified chart that displays both metric and imperial units side-by-side.

Flange Size Variations by Type

Flange sizes remain constant across face types, but thickness and other factors may vary slightly. For instance:

• Raised Face (RF) flanges add a small height (1.5–3 mm) to total flange thickness.
• Flat Face (FF) flanges do not have any elevation and match with flat gaskets.
• Neck flange or weld neck flange has a tapered neck that matches the pipe bore, requiring full match in NB and pressure class.

Understanding face types in conjunction with the flange size chart ensures that both mechanical and sealing compatibility is achieved.

Common Flange Size Ranges Used in India

In Indian projects, the most commonly used flange sizes include:

• ½ inch to 6 inch for building and water systems
• 3 inch to 8 inch for industrial applications
• 10 inch to 24 inch in refineries and large plants

Most engineers and buyers refer to ANSI B16.5 flange charts, which are accepted across piping contracts in India. At Udhhyog, we help clients select the exact flange size depending on pipe NB, wall thickness, and line pressure.

Real Use-Case: Confusion Between 2.5 Inch and 3 Inch Flange Sizes

A buyer in a Chennai-based textile plant once ordered 2.5-inch GI flanges assuming they’d fit their system. On arrival, the bolt holes didn’t align with the valves. Upon review, it was revealed that the pipeline actually used 3-inch flanges. This is a classic error due to flange NB confusion.

A quick reference to a flange size chart in mm would have shown that the BCD for 2.5 inch is 127 mm, while for 3 inch, it’s 152 mm. That 25 mm difference caused a complete mismatch — and a week-long delay.

Converting Flange Sizes Between MM and Inch

Most engineers face confusion when moving between Indian and US equipment, especially on sites where both metric and imperial components exist. That’s where a flange size chart showing both mm and inch becomes useful.

For example:

• 2 inch = NB 50 mm
• 3 inch = NB 80 mm
• 4 inch = NB 100 mm
• 6 inch = NB 150 mm
• 8 inch = NB 200 mm

Understanding this conversion avoids confusion when purchasing from catalogs or matching international machinery.

You can also read:

• How to Measure Flange Size – Expert Guide | Udhhyog
• Flange Dimensions Explained – OD, BCD, Thickness | Udhhyog

Common Mistakes Made While Using Flange Size Charts

• Ignoring the pressure class column and using NB alone
• Using OD to select flange without checking BCD or bolt count
• Forgetting face type thickness while matching gaskets
• Confusing inch-based and mm-based size charts
• Not verifying the flange material grade (SS, GI, MS)

Avoiding these mistakes requires discipline — and access to an accurate, standard-compliant flange size chart.

Why Udhhyog Flange Size Charts Are Trusted Across India

Udhhyog’s flange size charts are formatted for ease-of-use. Our charts:

• Include NB, OD, BCD, bolt data, and thickness
• Are available in both mm and inch format
• Are verified against ANSI, ASA, and IS standards
• Are available as downloadable PDFs for engineers
• Come with visual layout guidance upon request

From a fire-fighting line in Surat to a borewell project in Nagpur, our charts have helped hundreds of clients verify flange sizes remotely before placing orders.

Final Tips Before Using Flange Size Chart

• Verify the flange’s NB and pressure class
• Measure OD and BCD manually if replacing existing flange
• Confirm face type to match gasket correctly
• Use a standard-compliant chart (avoid outdated vendor tables)
• Always cross-check when converting between inch and mm

Still unsure? Contact the Udhhyog team with dimensions or a photo. We’ll help identify the exact flange and send a verified size chart in minutes.