Short answer. Pipe is ordered by nominal size (NPS, in inches, or DN in mm) plus a schedule or class that sets the wall thickness. The nominal size is not the real outside diameter: a 1/2 in pipe is 21.3 mm OD and a 2 in pipe is 60.3 mm OD. Schedule 40 is the everyday wall; Schedule 80 is thicker for higher pressure. GI pipe is the same steel pipe with a zinc coat, so the sizes are the same as BI.
Nominal size is not the outside diameter
A pipe's size name is a nominal size. For sizes up to 12 in, the outside diameter (OD) is fixed for each size and the schedule changes the wall, so the bore gets smaller as the wall gets thicker. That is why fittings for a given size fit every schedule of it.
The American standard for these dimensions is ASME B36.10M, used with pipe made to ASTM A53. In metric, the same sizes are called DN: DN15 is 1/2 in, DN20 is 3/4 in, DN25 is 1 in, DN50 is 2 in.
Schedule 40 and 80 chart
| Size (in) | DN | OD (mm) | Sch 40 wall (mm) | Sch 40 kg/m | Sch 40 per 6 m (kg) | Sch 80 wall (mm) | Sch 80 kg/m |
|---|---|---|---|---|---|---|---|
| 1/2 | 15 | 21.3 | 2.77 | 1.27 | 7.6 | 3.73 | 1.62 |
| 3/4 | 20 | 26.7 | 2.87 | 1.69 | 10.1 | 3.91 | 2.20 |
| 1 | 25 | 33.4 | 3.38 | 2.50 | 15.0 | 4.55 | 3.24 |
| 1-1/4 | 32 | 42.2 | 3.56 | 3.39 | 20.4 | 4.85 | 4.47 |
| 1-1/2 | 40 | 48.3 | 3.68 | 4.05 | 24.3 | 5.08 | 5.41 |
| 2 | 50 | 60.3 | 3.91 | 5.44 | 32.6 | 5.54 | 7.48 |
| 2-1/2 | 65 | 73.0 | 5.16 | 8.63 | 51.8 | 7.01 | 11.41 |
| 3 | 80 | 88.9 | 5.49 | 11.29 | 67.8 | 7.62 | 15.27 |
| 4 | 100 | 114.3 | 6.02 | 16.08 | 96.5 | 8.56 | 22.32 |
| 6 | 150 | 168.3 | 7.11 | 28.26 | 169.6 | 10.97 | 42.56 |
OD and walls from ASME B36.10M. Weight is plain-end black steel by the standard's formula: kg/m = (OD − wall) × wall × 0.02466. Galvanizing adds a little zinc weight; threads and couplings change it slightly. Schedule 40 matches "Standard" (STD) wall and Schedule 80 matches "Extra Strong" (XS) for these sizes.
British sizes: light, medium and heavy
Some GI pipe in the Philippines is made to the British system: the old BS 1387, now replaced by EN 10255. Instead of schedules it uses classes: light (L, often marked with a brown band), medium (M, blue band) and heavy (H, red band). The outside diameters follow ISO 65 and are close to, but not always the same as, the American sizes.
| Size (in) | DN | OD (mm) | Light wall (mm) | Medium wall (mm) | Heavy wall (mm) |
|---|---|---|---|---|---|
| 1/2 | 15 | 21.3 | 2.0 | 2.6 | 3.2 |
| 3/4 | 20 | 26.9 | 2.3 | 2.6 | 3.2 |
| 1 | 25 | 33.7 | 2.6 | 3.2 | 4.0 |
| 1-1/4 | 32 | 42.4 | 2.6 | 3.2 | 4.0 |
| 1-1/2 | 40 | 48.3 | 2.9 | 3.2 | 4.0 |
| 2 | 50 | 60.3 | 2.9 | 3.6 | 4.5 |
| 2-1/2 | 65 | 76.1 | 3.2 | 3.6 | 4.5 |
| 3 | 80 | 88.9 | 3.2 | 4.0 | 5.0 |
| 4 | 100 | 114.3 | 3.6 | 4.5 | 5.4 |
Walls from BS 1387 (carried into EN 10255 series L, M and H). The light class uses a slightly smaller OD in some sizes. Note 2-1/2 in: 76.1 mm OD in the British/ISO system vs 73.0 mm in ASME. The two do not take the same couplings.
Schedule or class: which one to ask for
- If the plans or the specification name a schedule (Sch 40, Sch 80), order to ASME / ASTM A53 sizes.
- If they name a class (BS 1387 medium, EN 10255 M), order the British series.
- For general water lines, Schedule 40 and BS medium have similar walls in the small sizes, but check the OD where your new pipe meets an existing line, especially at 2-1/2 in.
- Pressure ratings depend on the wall, the grade, the joints and the temperature; the design or plumbing code sets them, not the size alone.
Not sure whether you need GI or BI? Read GI pipe vs BI pipe.
Common sizes buyers ask us for
Lines that come up on buyers' lists. Examples, not a limit:
- GI pipe, Schedule 40 or Schedule 20: 1-1/2, 2, 2-1/2, 3, 4 and 6 in, in 6 m lengths
- Round steel tube (HSS) from 3 in to 16 in, for example 4.5 in OD × 0.188 in wall and 8.625 in OD × 0.50 in wall
- Steel casing: 610 mm OD × 6 mm × 6 m
- Ring plates for NPS 3, 4, 6 and 8 pipe: 6 mm A36, 50 mm wide, cut to the pipe OD
What to put in your RFQ
- GI or BI, and the standard if named (ASTM A53, BS 1387 or EN 10255).
- Nominal size and schedule or class.
- Length: full 6 m lengths, or cut lengths with the size of each piece.
- Ends: plain, bevelled for welding, or threaded (say NPT or BSPT). See threading and machining.
- Fittings for the same run: elbows, tees, couplings, nipples. See pipe fittings types.
- Quantity, delivery city and target date.
Add GI pipe or BI pipe to your RFQ and type the size and schedule. Not on our list? Send your size.
