How to Measure a Flange Gasket in Six Steps

Getting a flange gasket wrong by even a few millimetres can mean a leaking joint, a failed pressure test, or an unplanned shutdown. How to measure a flange gasket properly comes down to six checks: face type, outside diameter, inside diameter, bolt circle, bolt holes, and thickness. When you get this right, ordering the correct gasket, whether off the shelf or custom cut, becomes straightforward.

This guide walks through each measurement step, including the tools you need and a mistake to avoid.

Quick Answer

• You’ll need a tape measure, a vernier calliper, a notepad, and, ideally, the old gasket alongside the flange itself.
• Full-face gaskets require bolt-pattern measurements as well as OD, ID, and thickness. Ring gaskets do not have bolt holes, so the core physical measurements are OD, ID and thickness.
• If the old gasket is damaged or compressed, measure the flange face, bore and bolt pattern directly instead.
• Match your figures against a standard flange chart (ANSI, AS2129, AS4087, PN, or Table D/E/F/H), or send them through for a custom cut.

Tools You’ll Need

Before you start, gather:

• A steel tape measure or ruler
• A vernier calliper for bolt holes and thickness, more accurate than a tape on small dimensions
• A notepad or phone to record each measurement
• The old gasket, if you still have it, alongside the flange itself

How to Measure a Flange Gasket in Six Steps

Step 1: Identify the Flange Face and Gasket Style

Check whether the flange has a raised face, a flat face, or a ring-type joint (RTJ) groove, as this determines the gasket style you need. Flat-face flanges commonly use full-face gaskets, while raised-face flanges commonly use ring gaskets, including spiral-wound gaskets for higher-duty applications. RTJ flanges use a metal ring gasket seated in the machined groove.

Step 2: Measure the Outside Diameter (OD)

Lay your tape measure or calliper flat across the widest point of the gasket or flange face, edge to edge. If you’re measuring an old gasket, take two or three readings around the circumference. If the measurements vary noticeably, measure the flange face itself rather than relying on a gasket that has compressed or distorted.

Step 3: Measure the Inside Diameter (ID)

Measure across the centre bore, from inside edge to inside edge. The ID needs to clear the pipe bore without restricting flow, so check it against the flange’s nominal pipe size if you have it.

Step 4: Measure the Bolt Circle Diameter (BCD)

The BCD runs through the centre of every bolt hole. With an even number of holes, measure from the centre of one hole to the centre of the hole directly opposite. For an odd-number bolt pattern, use the flange specification or bolt-circle chart to confirm the BCD rather than estimating it from opposite holes.

Step 5: Count and Measure the Bolt Holes

Count the total number of bolt holes around the flange, then measure the diameter of one hole with your calliper. Bolt hole count and size are standard within a given flange rating, so this also works as a cross-check against your chart.

Step 6: Measure Thickness and Confirm the Material

Measure the gasket thickness with a calliper, checking a few points if the material has aged or compressed unevenly. Thickness affects compression and sealing pressure, so it needs to match the original spec rather than “close enough.” This is also the point to confirm the material: rubber, PTFE, compressed fibre and paper gaskets each suit different pressures, temperatures and media, so match the material to the job, not just the shape.

A Mistake Worth Avoiding

Don’t rely solely on a damaged or heavily compressed old gasket. If it’s brittle, torn, or squashed out of shape, measure the flange face, bore and bolt pattern directly instead, then work out the gasket size from there. It’s a far more reliable result than copying dimensions from a gasket that’s already failed.

Need a Gasket Cut to Your Exact Measurements?

If your measurements don’t match a standard chart or you’re working with a non-standard flange, our custom-made gasket service can cut one to your exact dimensions. Using in-house CNC cutting services, we replicate your measurements with tight tolerances for an accurate fit. 

Request a quote with your six measurements, and our team will confirm the right product and material for the job.

Shipping Australia Wide

As a Melbourne-based gasket manufacturer, Industrial Experts ships flange gaskets and custom-cut products Australia-wide. Once you know how to measure a flange gasket, getting the right one to your door is just as straightforward.

Stock gaskets ordered before 12pm AEST ship the same day from Melbourne, and most custom jobs go out within 1 to 2 days. From there, delivery timeframes depend on where you’re located. As a general guide:

Metro hubs tend to sit at the faster end of these ranges, while regional and more remote or mine site locations take longer. Treat these as a general guide rather than a fixed timeframe, since actual delivery depends on your exact location, stock availability, and courier schedules at the time of order.

Get the Right Fit, First Time

Whether you need a standard ring gasket or a custom-cut solution, our team can confirm your measurements before you order. Contact us on 0400 762 823 and request a sample to check the material before committing to a full order.