If you’ve ever sourced or worked with industrial piping systems, you’ve probably heard of BS flanges—one of the most widely used connection types for joining pipes, valves, pumps, and other equipment in sectors like oil and gas, water treatment, chemical processing, and power generation. As someone who’s spent over a decade as a BS flanges supplier, I often get asked the same question by new and repeat clients alike: “What are the actual dimensions of BS flanges?” It’s not a simple yes-or-no answer, and skimming a generic spec sheet won’t help you understand how those dimensions translate to on-site functionality, cost, and long-term reliability. Today, I’m breaking down the core dimensions of BS flanges, what each measurement means, why they matter, and what to keep in mind when sourcing your next order. BS Flanges
First, let’s set a quick baseline for anyone new to BS flanges. BS stands for British Standard, a set of specifications developed by BSI Group that standardized flanges in the UK and across much of the Commonwealth. Over time, these standards have evolved—BS 10 is the old general-purpose flange standard, BS 10 Table D is for pressure vessels, BS 4504 replaced the earlier BS 10 standards in 1969, and the most recent update for welded neck flanges is BS EN 1092-1, which aligned British standards with European norms while retaining the BS label familiar to generations of industrial buyers. But regardless of the specific BS standard in question, there are four non-negotiable dimensions that define every BS flange: nominal size, pressure rating, outside diameter, and bolt circle diameter. Let’s go through each one in detail.
Nominal size (often written as DN, or nominal diameter) is the first and most basic dimension. Think of this as the “generic size” of the flange, matched to the outside diameter of the pipe it connects to. A DN 100 flange, for example, is designed to fit a DN 100 pipe, which has an outside diameter of 114.3mm. Wait—you might notice that’s not a round number, and that’s intentional. Nominal size is not an exact measurement of the flange’s inner hole; it’s a standardized label that ensures interchangeability across manufacturers. So a DN 100 flange from any reputable BS supplier will fit a DN 100 pipe, no matter who made it, as long as it meets the standard. I can’t tell you how many times I’ve fielded a call from a contractor who ordered a “4-inch flange” (the imperial equivalent of DN 100) and got something that didn’t match their pipe because they didn’t specify DN or nominal size correctly. That’s why this is the starting point for every BS flange order.
Next up is pressure rating, another critical dimension often overlooked by new buyers. BS flanges use pressure ratings like PN 6, PN 10, PN 16, PN 25, PN 40, and PN 63—PN stands for “Nominal Pressure,” and the number is a rough approximation of the flange’s maximum pressure in bar at 20°C. The pressure rating directly impacts all other dimensions of the flange: a PN 16 DN 100 flange will be smaller than a PN 25 DN 100 flange, because higher pressure requires a thicker flange body, more bolts, and a wider sealing face to handle the force of fluid pushing against the connection. For example, a PN 16 DN 100 flange has an outside diameter of 220mm, while a PN 25 DN 100 flange has an outside diameter of 245mm—those 25mm extra millimeters make a huge difference when you’re connecting pipes that have to hold back high-pressure steam or corrosive chemicals. I always tell clients to never “size down” on pressure rating to save a few pounds; a flange that can’t handle the operating pressure of your system is a safety hazard waiting to happen.
Beyond nominal size and pressure rating, two key dimensions relate to how the flange is installed and sealed: outside diameter (OD) and bolt circle diameter (BCD). The outside diameter is exactly what it sounds like—the total width of the flange itself, from outer edge to outer edge. As I mentioned earlier, this varies by pressure rating: higher PN ratings mean larger OD, because the flange needs more material to resist bending or breaking under pressure. The bolt circle diameter is equally important: this is the diameter of the imaginary circle that passes through the center of each bolt hole on the flange. For a DN 100 PN 16 flange, the BCD is 180mm; for a DN 100 PN 25, it’s 185mm. Matching the BCD is non-negotiable for proper installation—if your bolts don’t line up with the holes on the adjacent flange, you can’t tighten the connection evenly, leading to leaks or even flange failure. I once had a client who tried to reuse old bolts from a decommissioned line, and the BCD was off by 5mm, resulting in a leak that shut down a water treatment plant for 12 hours. That’s a mistake that’s easy to avoid when you’re paying attention to BCD.
There are also specialized dimensions that come into play for specific types of BS flanges, which I see come up a lot when working with clients in niche industries. For example, sealing face dimensions: BS flanges have different sealing face types, each with its own measurement specifications. The most common is a flat face (FF), used for low-pressure applications, with a smooth sealing surface. For higher pressure or for use with toxic fluids, you might get a raised face (RF) flange, which has a 2mm high raised ring in the center of the sealing surface, designed to create a tighter seal with a gasket. There’s also male and female (M&F) flanges, tongue and groove (T&G), and ring-type joint (RTJ) flanges for extreme high-pressure and high-temperature applications like oil and gas drilling. Each of these has its own set of measurements—for example, an RTJ flange requires a groove cut into the sealing face, with specific width and depth dimensions to fit the metal ring gasket. I always make sure to ask clients what sealing face they need, because it’s a detail that’s often buried in spec sheets but critical for preventing leaks.
Another dimension that’s often overlooked is the flange thickness, or hub height, for welded neck flanges (the most common type of BS flange for permanent piping systems). Welded neck flanges have a tapered hub that connects the flange body to the pipe, which adds strength and reduces stress at the weld joint. The hub height isn’t random: it’s standardized by pressure rating, with PN 16 DN 100 flanges having a hub height of around 60mm, while PN 40 DN 100 flanges have a hub height of over 80mm. That extra height distributes the stress of the pipe’s pressure and thermal expansion away from the weld, which is why welded neck flanges are preferred over slip-on flanges for high-stakes applications. Slip-on flanges, by contrast, have a lower profile and are designed to be slipped over the end of a pipe and welded on both sides—they’re cheaper, but their lower thickness means they’re only suitable for low-pressure, low-temperature use. I always explain the difference between hub dimensions for each flange type to clients, so they don’t pay for a more expensive flange than they need, or buy a flange that’s underbuilt for their application.
Now, I should also address a common confusion that comes up with BS flange dimensions: imperial vs. metric. For decades, BS flanges were specified in both imperial units (inches, pounds per square inch) and metric units (millimeters, bar), and even today, some suppliers still reference both. For example, a 4-inch flange is the imperial equivalent of DN 100, and a 150-pound rating is roughly equivalent to PN 16. But it’s critical to not mix these up—an imperial 150-pound flange is not the same as a metric PN 16 flange, even though they’re close. I’ve had clients mix these units up more times than I can count, leading to flanges that are slightly too small or too large, requiring costly rework. That’s why I recommend always ordering BS flanges using DN and PN, the standardized metric system for British standards, to avoid conversion errors.
As a BS flanges supplier, I also know that dimensions aren’t just about spec sheets—they have real-world implications for cost, lead time, and compatibility with existing systems. If you’re replacing a flange in an old pipe system that was installed before the 1990s, it might not meet the latest BS EN 1092-1 standards. Some older BS 10 flanges have slightly different bolt hole patterns or sealing face dimensions, which means a modern PN 16 DN 100 flange won’t fit an old BS 10 Table E flange, even if they have the same nominal size. That’s why I always ask clients to send over a photo or a physical sample of the old flange if they’re doing a replacement, instead of just ordering by size—saving you the hassle of having to return a flange that doesn’t fit.
Another thing I always emphasize with new clients is that reputable BS flanges suppliers don’t cut corners on dimension tolerances. The standard allows for small variations, but those variations can add up to big problems. For example, a bolt hole that’s 1mm off-spec might not line up with your bolts, or a flange thickness that’s too thin might cause it to warp under pressure. That’s why when we manufacture our BS flanges, we follow strict BSI guidelines, with dimensional inspections at every step of production, from raw material cutting to final machining. We also provide a dimensional certificate with every order, so clients can verify that the flanges meet all required specs before installation.
Let’s also talk about a recent development in BS flange dimensions, for anyone working in sectors that handle cryogenic fluids or high-temperature gases. The latest update to BS EN 1092-1 added specific dimensions for flanges designed for extreme temperature ranges, from -196°C for liquefied natural gas to 800°C for power generation applications. These flanges have adjusted thickness and sealing face dimensions to accommodate thermal expansion and contraction, which regular BS flanges can’t handle. I’ve seen a lot of suppliers try to pass off standard PN flanges as temperature-rated, but that’s a mistake—always confirm the temperature class of your system with your supplier to get the right dimensions.
If you’re still feeling overwhelmed by all these dimensions, here’s a quick checklist to keep in mind when ordering BS flanges: 1) Confirm the nominal size (DN) and matching pipe outside diameter, not just the imperial “inch” size; 2) Specify the correct pressure rating (PN) based on your system’s maximum operating pressure; 3) Note the sealing face type (FF, RF, RTJ, etc.) for your application; 4) If replacing an old flange, provide a sample or photo to check for legacy BS 10 dimensions; 5) Ask for dimensional certification to verify compliance with BSI standards.
At the end of the day, BS flange dimensions aren’t just numbers on a page—they’re the foundation of a safe, reliable piping system. A flange that’s measured correctly fits right, seals tight, and lasts for years, while a flange that’s under-dimensioned or mismeasured can lead to costly leaks, unplanned shutdowns, and even safety incidents. As a supplier who’s been in this industry for years, I’ve seen every mistake from mixing units to skipping dimensional checks, and I always tell clients that investing in correctly dimensioned BS flanges is worth every penny.
If you’re sourcing BS flanges for your next project, whether it’s a small residential water system or a large-scale oil and gas facility, don’t leave the dimensional details to chance. We work with clients across all industries to provide custom and standard BS flanges that meet all BSI specifications, with precise dimensional measurements and full certification. Our team is always available to answer questions about dimensions, pressure ratings, and flange types, so you can order with confidence. Reach out today to discuss your requirements, and we’ll help you find the right flanges for your application.
Pipe Fittings References
British Standards Institution. BS 4504-1:1989: Metallic pipe flanges for pipelines, pressure vessels and fittings. Specification for flanges with nominal pressures PN 2, PN 5, PN 10, PN 16, PN 25, PN 40 and PN 63.
British Standards Institution. BS EN 1092-1:2007: Flanges and their joints. Circular flanges for pipes, valves, fittings and accessories, PN designated. Steel flanges.
BSI Group. Guide to BS Flange Specifications: Dimensions, Pressure Ratings and Application Notes. 2022.
Hebei Haihao Group Huadian High Pressure Pipe Fittings Co., Ltd.
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Address: Donglin Industrial Zone, Mengcun County, Cangzhou City, Hebei Province, China
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