Pipe size chart: DN, NPS and outside diameter
DN 65, 2½ inches, 76.1 or 73 mm: almost the same size, but not quite. Here is how to read pipe sizes so the fitting you order really matches the pipe.
Three names for the same pipe
On a drawing, a purchase order or a stock list, the same line may be called “DN 100”, “4 inch” or “114.3”. They all describe the same pipe, but they do not say the same thing:
- DN (nominal diameter, ISO 6708) is a dimensionless reference, close to the bore but not equal to it.
- NPS (Nominal Pipe Size, in inches) is the American equivalent used by ASME standards.
- The outside diameter, in millimetres, is the only real, measurable dimension. It is what matters for a butt-weld fitting: both ends must line up.
Wall thickness comes next. “114.3 × 3.6” means a 114.3 mm outside diameter with a 3.6 mm wall. In ASME terms the same size would read “4" STD” or “4" Sch 40”.
The size chart
European (EN 10220, used by EN 10253) and American (ASME B36.10, used by ASME B16.9) outside diameters are the same for most sizes. Those that differ are in bold.
| DN | NPS (inch) | EN series OD (mm) | ASME series OD (mm) |
|---|---|---|---|
| DN 15 | ½" | 21.3 | 21.3 |
| DN 20 | ¾" | 26.9 | 26.7 |
| DN 25 | 1" | 33.7 | 33.4 |
| DN 32 | 1¼" | 42.4 | 42.2 |
| DN 40 | 1½" | 48.3 | 48.3 |
| DN 50 | 2" | 60.3 | 60.3 |
| DN 65 | 2½" | 76.1 | 73.0 |
| DN 80 | 3" | 88.9 | 88.9 |
| DN 100 | 4" | 114.3 | 114.3 |
| DN 125 | 5" | 139.7 | 141.3 |
| DN 150 | 6" | 168.3 | 168.3 |
| DN 200 | 8" | 219.1 | 219.1 |
| DN 250 | 10" | 273.0 | 273.0 |
| DN 300 | 12" | 323.9 | 323.8 |
EN 10220 and ASME B36.10 series.
The five sizes that do not match
For DN 20, 25 and 32 the gap is 0.2 to 0.3 mm: small, but enough for a slight step at the bevel on a careful line. Two sizes are clearly different:
- DN 65 / 2½": 76.1 mm in Europe, 73.0 mm in ASME, more than 3 mm apart. A 73.0 elbow cannot be welded properly to a 76.1 pipe.
- DN 125 / 5": 139.7 mm in Europe, 141.3 mm in ASME.
Rule of thumb: always give the outside diameter and wall thickness in millimetres on an order, not just DN or inches. It is the only way to avoid mistakes on these five sizes.
Reading a fitting description
A few examples from our catalog:
- 90° 3D elbow 114.3 × 3.6 S235: 90° elbow with a 3D radius, 114.3 mm OD, 3.6 mm wall, S235 steel (EN 10253-1).
- 90° 1.5D elbow 4" STD A234 WPB: 4-inch long radius elbow (114.3 mm OD), standard wall, A234 WPB steel (ASME B16.9).
- Reducer 114.3 × 3.6 – 76.1 × 2.9: the large end always comes first, then the small end.
Ordering without mistakes
- Fitting type and angle or form (90° 3D elbow, eccentric reducer…).
- Outside diameter and wall thickness, at both ends for a reducer.
- Standard and grade: EN 10253-1 in S235, or ASME B16.9 in A234 WPB.
- Finish: black, galvanized or painted.
- Quantity per reference (50 pieces minimum with us) and the certificate you need.
Our catalog references already carry all of this: add them to your quote list and there is nothing to retype.
Frequently asked questions
Is DN 100 the same as 4 inch?
Yes. DN 100 and 4" both describe a pipe with a 114.3 mm outside diameter, in the European and in the ASME series.
Can I weld an ASME fitting to a European pipe?
Yes where the outside diameters are identical (21.3, 48.3, 60.3, 88.9, 114.3, 168.3, 219.1, 273 mm), provided the wall thicknesses are compatible. No for DN 65 and DN 125, where the diameters clearly differ.
Is DN the inside diameter?
No. It is a rounded designation close to the bore. The real inside diameter depends on the wall thickness.