Butt-weld fitting wall thickness: schedules, STD, XS and millimetres
A butt-weld fitting takes the wall of the pipe. The rule is simple; applying it is less so, between schedules, ASME weight classes and European walls in millimetres.
The fitting takes the wall of the pipe
A butt weld joins two walls face to face. A fitting thinner than the pipe becomes the weak point of the line; a thicker one puts the bores out of line, and the weld root becomes hard to make and to inspect. The basic rule is therefore: same outside diameter, same wall.
During forming, wall thickness varies along the part, especially on an elbow. ASME B16.9 does not fix the body thickness of the fitting, but requires that it is nowhere less than 87.5% of the nominal wall ordered. That is the same tolerance as for ASME pipe.
Schedule, STD and XS: two overlapping systems
ASME B36.10M uses two systems. Schedules (Sch 10, 20, 30, 40, 60, 80, 100, 120, 140, 160) are a numerical series: the higher the number, the thicker the wall for a given size. The weight classes STD (standard), XS (extra strong) and XXS (double extra strong) are older and remain the most common reference for fittings.
The two systems overlap over part of the range: STD has the same wall as Sch 40 up to 10", XS the same as Sch 80 up to 8". Above that they part ways. At 12" an STD fitting is 9.53 mm while a Sch 40 pipe is 10.31 mm; at 10" XS is 12.70 mm and Sch 80 15.09 mm.
ASME wall thickness, ½" to 12"
| NPS | OD (mm) | STD | XS | Sch 40 | Sch 80 |
|---|---|---|---|---|---|
| ½" | 21.3 | 2.77 | 3.73 | 2.77 | 3.73 |
| ¾" | 26.7 | 2.87 | 3.91 | 2.87 | 3.91 |
| 1" | 33.4 | 3.38 | 4.55 | 3.38 | 4.55 |
| 1¼" | 42.2 | 3.56 | 4.85 | 3.56 | 4.85 |
| 1½" | 48.3 | 3.68 | 5.08 | 3.68 | 5.08 |
| 2" | 60.3 | 3.91 | 5.54 | 3.91 | 5.54 |
| 2½" | 73.0 | 5.16 | 7.01 | 5.16 | 7.01 |
| 3" | 88.9 | 5.49 | 7.62 | 5.49 | 7.62 |
| 4" | 114.3 | 6.02 | 8.56 | 6.02 | 8.56 |
| 5" | 141.3 | 6.55 | 9.53 | 6.55 | 9.53 |
| 6" | 168.3 | 7.11 | 10.97 | 7.11 | 10.97 |
| 8" | 219.1 | 8.18 | 12.70 | 8.18 | 12.70 |
| 10" | 273.0 | 9.27 | 12.70 | 9.27 | 15.09 |
| 12" | 323.8 | 9.53 | 12.70 | 10.31 | 17.48 |
Wall thickness in mm to ASME B36.10M. In bold: sizes where Sch 40 and STD, or Sch 80 and XS, no longer match.
EN 10253-1 wall thickness
EN 10253-1 fittings are designated directly by outside diameter and wall in millimetres. They go with structural and distribution pipe (water, heating, fire protection, compressed air), whose walls are much thinner than the ASME series. The walls of our S235 range:
| OD (mm) | S235 wall (mm) | Families made |
|---|---|---|
| 21.3 | 2.0 | caps, elbows |
| 26.9 | 2.3 | caps, elbows, reducers |
| 33.7 | 2.6 | caps, elbows, reducers |
| 42.4 | 2.6 | caps, elbows, reducers |
| 48.3 | 2.6 | caps, elbows, reducers |
| 60.3 | 2.9 | caps, elbows, reducers, tees |
| 76.1 | 2.9 | caps, elbows, reducers, tees |
| 88.9 | 3.2 | caps, elbows, reducers, tees |
| 114.3 | 3.6 | caps, elbows, reducers, tees |
| 139.7 | 4.0 | caps, elbows, reducers, tees |
| 168.3 | 4.5 | caps, elbows, reducers, tees |
| 219.1 | 6.3 | caps, elbows, reducers, tees |
| 273.0 | 6.3 | elbows |
| 323.9 | 7.1 | elbows |
EN 10253-1 range of our catalog.
At the same diameter, the two ranges do not share the wall: 114.3 × 3.6 in EN 10253-1 against 6.02 mm (STD) or 8.56 mm (XS) in ASME B16.9. The parts are not interchangeable on one line.
Choosing the wall
A fitting’s wall is not calculated separately: it follows the pipe, which the design office sizes under the applicable piping code (pressure, temperature, corrosion allowance). ASME B16.9 treats a fitting as having the same allowable pressure as straight seamless pipe of the same grade and wall. In practice:
- ASME line in Sch 40 up to 10": STD fittings;
- ASME line in Sch 80 up to 8": XS fittings;
- line in EN 10220 or EN 10255 pipe: EN 10253 fittings at the pipe wall.
When the wall does not match
Sometimes no fitting exists at the exact pipe wall. Piping codes such as ASME B31.3 then allow a machined taper on the thicker part, bringing it down to the other wall at the weld. The design office must approve it and show it on the drawing.
Common mistakes
- Ordering 12" "Sch 40" and expecting STD, or the other way round.
- Pairing an EN fitting in S235 with an ASME pipe of the same diameter without comparing walls.
- Giving only the DN on an order, with no wall or schedule.
Standards and references
- ASME B36.10M: steel pipe walls (schedules, STD, XS, XXS).
- ASME B16.9: minimum fitting wall (87.5% of nominal).
- ASME B31.3: transitions between unequal walls.
- EN 10253-1: non-alloy steel fittings, walls in mm.
Frequently asked questions
Is STD the same as Sch 40?
Up to 10", yes. From 12", Sch 40 is thicker than STD (10.31 mm against 9.53 mm).
What does XS mean?
Extra Strong, the class above STD. It matches Sch 80 up to 8".
Do EN 10253-1 walls have a schedule equivalent?
No. They follow European series in millimetres. For a line in schedules, choose ASME B16.9 fittings.
Is an elbow’s wall uniform?
No, it varies with forming. The standard guarantees a minimum at any point: 87.5% of nominal.
