Flange dimensions for export are where a surprising number of orders quietly go wrong: the outside diameter matches, the buyer goes to bolt it up, and the holes don't line up. Two flanges can share the same outside diameter and still have completely different bolt circles — which means "8-inch flange" tells your buyer almost nothing on its own. Ship the wrong drilling to a project overseas and the part comes back, freight both ways, on your account.
This is the set of flange dimensions export buyers actually ask for, why each one decides fit, and how to put them on your product images so the buyer confirms compatibility before the PO — not after the container lands.
What flange dimensions do buyers actually need?
A pipe flange is a disc that bolts two pipe ends (or a pipe and a valve) together, sealed at the joint. "How big is it" is never one number. A buyer needs seven, and leaving any one off is an invitation to a wrong-part claim:
| Dimension | Why it decides fit |
|---|---|
| Nominal size (NPS or DN) | The pipe bore it mates to — NPS 8 / DN 200 |
| Pressure rating (Class, PN, or K) | Sets thickness and drilling — a Class 150 won't mate a Class 300 |
| Outside diameter (O) | Overall disc size — necessary but not sufficient |
| Bolt circle diameter (BCD) | The circle the bolt-hole centers sit on — must match exactly |
| Number of bolt holes (n) | Must match the mating flange |
| Bolt hole diameter | Which bolt size passes through |
| Flange thickness (C) + face type | Sealing face and gasket seating |
The one buyers most often get burned by is the bolt circle. As the reference tables put it, the bolt circle "must match the mating flange exactly, even when two flanges share the same outside diameter." That single line is the reason a spec sheet beats a size label.
Why don't two flanges of the "same size" fit together?
Because "size" collapses three independent things — nominal bore, pressure class, and standard — into one word. Change any one and the drilling changes.
An 8-inch flange in ASME B16.5 Class 300 has a bolt circle of 270 mm (10.62 in) with 12 bolts; the same nominal 8-inch in Class 150 has a smaller bolt circle and only 8 bolts. Same "8 inch," different holes, will not mate. That is not an error — it is the standard doing its job of matching flanges to a pressure rating. Your listing has to say which one, or the buyer is guessing.
Getting the class wrong ships a physically-correct, spec-wrong part — it passes your QC, fails at the buyer's site, and comes back as a return. Before you decide "close enough," it helps to price the round trip: a mismatched flange class on an export order means a cross-border return, which you can size with the return cost calculator — freight, re-inspection, and re-stocking usually dwarf the part.
ASME, EN (DIN), or JIS — which flange standard does my buyer use?
This is a region question, and getting it wrong makes every other number meaningless. The three families are not interchangeable:
| Standard | Region | Rating system | Example ratings |
|---|---|---|---|
| ASME/ANSI B16.5 | US, Middle East, much of the Americas | Class (lb) | 150, 300, 600, 900, 1500, 2500 |
| EN 1092-1 (formerly DIN) | Europe | PN (bar) | PN 6, 10, 16, 25, 40 |
| JIS B2220 | Japan, parts of Asia | K | 5K, 10K, 16K, 20K |
ASME B16.5 alone covers NPS ½ through 24 in seven pressure classes. A PN 16 and a Class 150 are close in pressure but drilled differently — a European buyer sending you a PN 16 valve needs a PN 16 companion flange, not "the American equivalent." State the standard first, then the rating; the same discipline you'd apply to any pipe and fitting dimensions.
What do Class 150, PN 16, and 10K actually mean?
They are pressure-temperature ratings, not thicknesses you pick for looks. Class 150 carbon steel (A105) is rated about 285 psi at 100°F; Class 300 about 740 psi — more than double, with a thicker flange and a different, non-interchangeable bolt pattern. PN 16 means a nominal 16 bar rating; JIS 10K a roughly 10 kgf/cm² class. The buyer's system pressure sets the rating, and the rating sets the drilling. You do not get to substitute a lighter class to save steel — it changes the holes.
Which flange face type should I show — RF, FF, or RTJ?
Face type is part of the dimension story because it changes the sealing surface and the gasket. The three common ones:
- RF (raised face) — the most common; a small raised ring the gasket seats on.
- FF (flat face) — full-flat, used against cast-iron equipment to avoid cracking it.
- RTJ (ring-type joint) — a grooved face for a metal ring, for high pressure and temperature.
An RF flange and an FF flange of the same size and class still won't make a proper joint. Put the face type on the image next to the class — it takes four characters and prevents a return.
How should I put flange dimensions on a product image?
Not as a paragraph the buyer has to decode. The fix is to lock the measured numbers onto the photo where the eye already goes: the standard and rating as a header (e.g. "ASME B16.5 · Class 300 · RF"), then a small dimensioned inset showing OD, bolt circle, hole count, and thickness — the exact figures, not eyeballed ones.
This is where a measured annotation tool matters and an AI image generator actively hurts. An AI tool can render a clean flange photo, but it will happily print a bolt circle that looks plausible and is wrong, because it has no idea what the real part measures. A dimension has to come from the actual measured geometry — snap the measurement to the real edge, set the true value once, export at your catalog's image size. One accurate flange image then carries the same trustworthy numbers into your Alibaba listing, your PDF catalog, and your quotation. The broader discipline is the same as any time you label metal product specifications: the value is that every figure is true.
Flange spec checklist
Put every one of these on the listing before it goes live.
- Standard named first (ASME B16.5 / EN 1092-1 / JIS B2220)
- Pressure rating (Class / PN / K)
- Nominal size (NPS and DN, both units)
- Outside diameter (O)
- Bolt circle diameter (BCD) — the number that must match exactly
- Number of bolt holes and bolt hole diameter
- Flange thickness and face type (RF / FF / RTJ)
- All figures identical to the spec sheet — zero drift
FAQ
What dimensions are needed to identify a flange?
You need the standard (ASME/EN/JIS), the pressure rating (Class/PN/K), the nominal size (NPS or DN), the outside diameter, the bolt circle diameter, the number and diameter of bolt holes, and the face type. The bolt circle and bolt count are what actually decide whether two flanges mate.
Why don't two 8-inch flanges fit together?
Because nominal size doesn't fix the drilling. An 8-inch Class 150 and an 8-inch Class 300 have different bolt circles and different bolt counts, so the holes don't line up. Same for an ASME flange versus a PN-rated EN flange of similar size — different standards, different drilling.
Is Class 150 the same as PN 16?
No. They are close in pressure but come from different standards (ASME vs EN) with different bolt patterns and are not interchangeable. Match the buyer's standard, not just the pressure — a PN 16 system needs a PN 16 flange.
How do I show flange dimensions to an overseas buyer?
Put the standard and rating as a text header on the product image, then a dimensioned inset with OD, bolt circle, hole count, and thickness in real measured values, dual unit. A measured annotation on the photo beats a wall of text and beats an AI-rendered image that can invent the wrong bolt circle.