DN ↔ NPS: same name, sometimes a different pipe
DN and NPS are naming systems, not measurements — and the actual outside diameter behind the same DN differs between the American (ASME B36.10) and European (EN 10220/10255) series at several sizes. The flagged rows are the classic procurement traps: a DN65 flange set ordered across the Atlantic will not fit.
| DN | NPS | ASME OD (mm) | EN OD (mm) | Δ (mm) |
|---|---|---|---|---|
| 6 | NPS 1/8 | 10.29 | 10.2 | ≈ |
| 8 | NPS 1/4 | 13.72 | 13.5 | -0.2 ⚠ |
| 10 | NPS 3/8 | 17.14 | 17.2 | ≈ |
| 15 | NPS 1/2 | 21.34 | 21.3 | ≈ |
| 20 | NPS 3/4 | 26.67 | 26.9 | +0.2 ⚠ |
| 25 | NPS 1 | 33.40 | 33.7 | +0.3 ⚠ |
| 32 | NPS 1-1/4 | 42.16 | 42.4 | +0.2 ⚠ |
| 40 | NPS 1-1/2 | 48.26 | 48.3 | ≈ |
| 50 | NPS 2 | 60.32 | 60.3 | ≈ |
| 65 | NPS 2-1/2 | 73.02 | 76.1 | +3.1 ⚠ |
| 80 | NPS 3 | 88.90 | 88.9 | ≈ |
| 100 | NPS 4 | 114.30 | 114.3 | ≈ |
| 125 | NPS 5 | 141.30 | 139.7 | -1.6 ⚠ |
| 150 | NPS 6 | 168.27 | 168.3 | ≈ |
| 200 | NPS 8 | 219.07 | 219.1 | ≈ |
| 250 | NPS 10 | 273.05 | 273.0 | ≈ |
| 300 | NPS 12 | 323.85 | 323.9 | ≈ |
DN65 is the worst offender: 73.0 mm American vs 76.1 mm European. DN8, DN20, DN25, DN32 and DN125 also diverge enough to matter for clamps, couplings and rollers.