EN 10255 vs ASTM A53: why 6″ is not the same size in both standards
Short answer: EN 10255 is a European standard sized by DN nominal bore; ASTM A53 is sized by NPS to ASME B36.10. DN150 has a 165.1 mm outside diameter, 6″ NPS has 168.3 mm — so the two pipes are not interchangeable in the same coupling, and the threads are different as well (ISO 7/1 vs NPT).
Side by side
| EN 10255 | ASTM A53 | |
|---|---|---|
| Size system | DN nominal bore | NPS (inch) to ASME B36.10 |
| Wall thickness series | Light / medium / heavy | Schedule 10 / 40 / 80 (and others) |
| Typical threaded ends | ISO 7/1 taper, ISO 228 parallel | NPT to ASME B1.20.1 |
| Coating standard | EN 10240 (Class A.1 hot-dip) | No coating in A53 itself — see A123 / A153 |
| Ends supplied | Plain end, threaded & socketed with coupling | Plain end, threaded, grooved, bevelled |
| Where it is used | Europe, Middle East, Africa, water and HVAC lines | A53 is a common US / Gulf project standard |
The size chart that causes the trouble
Outside diameter in mm. Where the last column says “differs”, the pipe is not a drop-in replacement for the other standard — the coupling, the thread and the fittings all change with it.
| DN (EN 10255) | OD (mm) | Nearest NPS | OD of that NPS (mm) | Interchangeable? |
|---|---|---|---|---|
| DN15 | 21.3 | 1/2″ | 21.3 | Same OD |
| DN20 | 26.9 | 3/4″ | 26.7 | Differs by 0.2 mm |
| DN25 | 33.7 | 1″ | 33.4 | Differs by 0.3 mm |
| DN32 | 42.4 | 1¼″ | 42.2 | Differs |
| DN40 | 48.3 | 1½″ | 48.3 | Same OD |
| DN50 | 60.3 | 2″ | 60.3 | Same OD |
| DN80 | 88.9 | 3″ | 88.9 | Same OD |
| DN100 | 114.3 | 4″ | 114.3 | Same OD |
| DN125 | 139.7 | 5″ | 141.3 | Differs by 1.6 mm |
| DN150 | 165.1 | 6″ | 168.3 | Differs by 3.2 mm |
Read that table twice if your project mixes a European specification with a US one. Half of the sizes coincide, which is exactly why the other half catches buyers out — the pipe looks right until the thread will not take, or the coupling needs a different size.
How to order so it cannot go wrong
Order by outside diameter and wall thickness, not by nominal size alone: “OD 165.1 mm × 4.5 mm” is unambiguous, “6 inch” is not.
State the standard and the wall series: EN 10255 medium, or A53 Schedule 40. The same nominal pipe in light and heavy wall differs by more than a millimetre.
State the thread standard: ISO 7/1 or NPT. If the pipe is threaded and socketed, say whether the coupling should be included.
State the zinc weight you need. A customer who wants 500 g/m² and receives 300 g/m² has a rejection on hand, not a delivery.
Ask for the mill certificate with the measured zinc weight per lot — it is the document your inspector will check at loading.
Keep pipe and fittings on the same standard. Mixing ISO 7/1 pipe with NPT elbows is a field problem, not a warehouse problem.
What we supply
| Product | Standard | Size range | Wall / coating |
|---|---|---|---|
| Round galvanized pipe | EN 10255 | DN8 – DN150 | Light / medium / heavy wall, hot-dip 350–500 g/m² |
| Round galvanized pipe | ASTM A53 | ½″ – 8″ | Schedule 40 / 80, hot-dip galvanized |
| Threaded & socketed lengths | ISO 7/1 or NPT | DN15 – DN100 | Threads cut and gauged, coupling fitted, plastic caps |
| Square & rectangular tube | EN 10219 / A500 | 20×20 – 100×100 and 40×80 etc. | Pre-galvanized or hot-dip on request |
| Scaffolding pipe | EN 39 / EN 10219 | 48.3 × 3.2 / 4.0 mm | 6 m lengths or cut to order |
Questions we answer every week
Can I use EN 10255 pipe with NPT fittings?
No — not reliably. EN 10255 pipe is normally threaded to ISO 7/1 or ISO 228, A53 pipe to NPT. The thread forms differ, and an ISO 7/1 male thread will not seal in an NPT female fitting. State the thread standard on the order and keep pipe and fittings on the same one.
Is DN150 the same as 6 inch?
Close, but not the same: DN150 to EN 10255 is 165.1 mm OD, 6″ NPS to ASME B36.10 is 168.3 mm OD. Some sizes do coincide (DN80 and 3″, DN100 and 4″), which is why the exception is easy to miss.
What zinc coating weight should the certificate show?
Our standard is 350–500 g/m², with the measured zinc weight stated on the mill certificate for each lot so it can be checked against the specification before loading. If your project calls for a different figure, tell us at enquiry stage.
Which standard should I buy for an African or Middle East project?
Whichever the project specification names. If the specification is silent, EN 10255 is the more common choice for water, HVAC and general lines in African and Gulf markets, and A53 where the project follows US standards. We supply both from the same Tianjin base.
Send your sizes — price within 24 hours
Give us the standard (EN 10255 or A53), outside diameter or DN, wall thickness series, length, thread standard and quantity. If you only have a purchase history or a BOM, send that — we quote from the document.