How to Measure Flange Size – A Complete Guide for Engineers & Buyers

Understanding how to measure flange size is not just a technical skill — it's a practical necessity for anyone involved in the planning, procurement, or installation of piping systems. From factory pipelines to borewell installations, an incorrectly sized flange can delay operations, cause system leaks, or even force entire sections to be reworked. At Udhhyog, we’ve seen this happen firsthand — and we’ve helped hundreds of project heads avoid costly mistakes by simply getting their flange measurements right.

What is Flange Sizing?

Flange sizing refers to the process of determining the standard dimensions of a flange so that it fits perfectly with its corresponding pipe and system pressure. Most people think flange size means just the outer diameter or the pipe size, but in reality, several dimensions come into play. These include the outer diameter (OD), nominal bore (NB), bolt circle diameter (BCD), number of bolt holes, bolt hole diameter, flange face thickness, and the pressure class.

Every flange, whether it's stainless steel, mild steel, or galvanized iron, is built according to a standard — ANSI, ASA, IS, or DIN — and measuring it accurately ensures that it aligns with gaskets, bolt holes, valves, and adjoining pipeline sections.

Why Flange Measurement Is Critical in Projects

Incorrect flange sizing can lead to major technical issues. The most common problems include misaligned bolt holes, uneven gasket compression, weak joint sealing, or pressure mismatches. In industrial setups, these issues translate into mechanical failure, downtime, and financial losses.

For example, a maintenance team in a dairy plant once attempted to replace a 2-inch slip-on flange with a 2-inch weld neck flange, assuming both were interchangeable. The bolt hole pattern and thickness were different, leading to poor sealing and a subsequent pipeline shutdown. A simple measurement check beforehand could have prevented the issue.

Tools Required for Flange Measurement

You don’t need complex instruments to measure a flange. A basic measuring tape, vernier caliper, and a flange size chart are often all you need. For tighter spaces, a mirror or flashlight helps inspect bolt holes and markings. On older installations, where markings are faded or corroded, a simple scale or flexible thread may be used to trace dimensions before cross-referencing with a flange standard.

Step-by-Step: How to Measure Flange Size

To measure a flange accurately:

1. First, measure the outer diameter. This is the full diameter from edge to edge of the flange face. It’s your first reference point.
2. Second, identify the nominal bore (NB) — the inner hole of the flange that matches the pipe diameter. NB is standardised in mm or inches.
3. Third, measure the bolt circle diameter (BCD) — this is the distance across the flange face through the center of opposite bolt holes. This measurement is critical for aligning bolt connections.
4. Fourth, count the number of bolt holes. Flanges come with a specific number of bolt holes for each pressure class and pipe size.
5. Fifth, measure the bolt hole diameter using a vernier caliper. This ensures your bolts fit correctly without movement or stress.
6. Sixth, measure the thickness of the flange body. This contributes to the strength and determines its pressure-handling capability.
7. Finally, verify the pressure class. If it's not clearly marked, it can often be identified by comparing the OD, thickness, and bolt pattern with a standard flange chart.

Raised Face vs Flat Face – How Face Type Affects Measurement

Most buyers overlook the impact of flange face types on overall measurement. Raised face (RF) flanges have a slight elevation at the sealing surface, which is not present in flat face (FF) flanges. This additional height (typically 1.5 to 3 mm depending on class) must be considered when determining total thickness, especially for gasket matching. RTJ flanges, or ring-type joint flanges, also feature a machined groove and are used in high-pressure systems. If you're measuring flange thickness and planning gasket alignment, always identify whether it's RF, FF, or RTJ.

Difference Between NB and OD – A Common Confusion

A frequent error is confusing Nominal Bore (NB) with Outer Diameter (OD). NB refers to the pipe size the flange fits. For example, a 2-inch flange fits a 2-inch NB pipe, but its OD might be 150 mm or more. OD is the physical width of the flange face, not the pipe. Buyers who only refer to OD without confirming NB may end up ordering the wrong size altogether.

Another confusion arises when people refer to “3-inch flange” but forget to mention whether it's Class 150 or Class 300. Both have different bolt hole patterns and thicknesses even though NB remains the same.

Real-World Example: Pharma Installation in Maharashtra

A pharmaceutical unit in Pune was replacing outdated flanges in a water distribution header. The original setup used 3-inch Class 300 flanges, but the buyer accidentally ordered Class 150 variants based on visual inspection alone. After installation, the system showed signs of vibration and leakage at peak pressure. Post-analysis revealed the mistake, and Udhhyog helped them measure and re-order the correct flanges using our chart and phone-assisted walkthrough.

Flange Measurement in Older Installations

Many project engineers face difficulty measuring flanges in older or corroded plants. Paint, rust, or mechanical damage may hide key markings. In such cases, the best way is to measure manually and refer to a flange chart. Use a mirror to access backside bolt holes. Clean the surface to expose numbers if possible. Measure thickness at multiple points if corrosion is uneven. Udhhyog also accepts client photos with basic dimensions and offers remote verification.

When to Use a Flange Size Chart

A flange size chart is your best friend when dealing with partial measurements or verifying ambiguous markings. Each standard chart lists NB size, OD, BCD, bolt hole size, bolt count, and thickness — matched to pressure ratings like Class 150, 300, or 600. By matching two or three known parameters like OD and bolt circle, you can identify the correct flange even if other details are missing.

Let’s say you have a flange with an OD of 152 mm and 4 bolt holes spaced 120 mm apart. A quick reference to ANSI B16.5 chart confirms it's a 2-inch Class 150 flange. Udhhyog shares these charts on request in printable PDF format.

Measuring Flange Size Without Removing It

Sometimes you need to replace or reorder a flange without removing the existing one. In that case:

• Measure OD with an external scale.
• Use a torch and mirror to count bolt holes.
• Measure BCD from center of one bolt to the opposite using thread if space is tight.
• Estimate thickness with a depth scale if front face is visible.
• If possible, photograph the entire setup and email it to Udhhyog. We assist with identification remotely and confirm dimensions with a chart.

Final Tips Before Ordering Flanges

Always verify OD, NB, BCD, number of bolt holes, and face type before placing an order. Share actual measurements or request a chart. Avoid generic terms like “3-inch flange” without pressure rating. Mention whether it’s slip-on, weld neck, or blind. Always mention face type: RF, FF, or RTJ. If unsure, send a photo or dimension list to Udhhyog’s technical team — we’ll confirm it before you order.