Pipe Schedule Chart Explained: SCH 10, 40, 80, 160 and XXS (NPS vs DN, Wall, Weight)
Pipe Schedule Chart Explained: SCH 10, 40, 80, 160 and XXS (NPS vs DN, Wall, Weight)
A steel pipe is described by two numbers that are not what they look like. "2-inch pipe" is not 2 inches across, and "schedule 40" is not a thickness. Here is how the system works, with the wall thicknesses and weights for the sizes that come up every day, straight from ASME B36.10M.
Rule 1: the outside diameter is fixed per size
Every nominal pipe size (NPS) has one outside diameter, whatever the wall thickness. That is what lets fittings, threads and clamps fit every schedule of the same size. Up to NPS 12 the outside diameter is larger than the nominal number; from NPS 14 upward the nominal size is the outside diameter in inches.
- NPS 1/2 = DN 15 → OD 21.3 mm
- NPS 1 = DN 25 → OD 33.4 mm
- NPS 2 = DN 50 → OD 60.3 mm
- NPS 3 = DN 80 → OD 88.9 mm
- NPS 4 = DN 100 → OD 114.3 mm
- NPS 6 = DN 150 → OD 168.3 mm
- NPS 8 = DN 200 → OD 219.1 mm
- NPS 12 = DN 300 → OD 323.8 mm
- NPS 14 = DN 350 → OD 355.6 mm (= 14.00 in)
- NPS 24 = DN 600 → OD 610.0 mm (= 24.00 in)
DN (diamètre nominal, EN 10220 / ISO 6708) is simply the metric name for the same size series — DN 50 and NPS 2 are the same pipe.
Rule 2: the schedule sets the wall
The schedule number is a rough pressure-to-stress ratio from the 1930s (Sch ≈ 1000 × P/S), which is why it is not a dimension. In practice it is just a label for a wall thickness that changes with size: schedule 40 is 3.91 mm on a 2-inch pipe and 8.18 mm on an 8-inch pipe. Higher schedule, thicker wall, same outside diameter, smaller bore.
The letter grades are older still: STD (standard), XS (extra strong) and XXS (double extra strong). STD equals schedule 40 up to NPS 10; XS equals schedule 80 up to NPS 8. Above that they diverge: STD is fixed at 9.53 mm (3/8 in) and XS at 12.70 mm (1/2 in) for every larger size, while the schedule numbers keep growing.
NPS 2 (DN 50, OD 60.3 mm) — every schedule
Weight per meter is (OD − t) × t × 0.02466 for steel, the EN 10220 / B36.10M plain-end formula.
- SCH 10 — wall 2.77 mm · bore 54.8 mm · 3.93 kg/m (2.64 lb/ft)
- SCH 40 / STD — wall 3.91 mm · bore 52.5 mm · 5.44 kg/m (3.65 lb/ft)
- SCH 80 / XS — wall 5.54 mm · bore 49.2 mm · 7.48 kg/m (5.03 lb/ft)
- SCH 160 — wall 8.74 mm · bore 42.8 mm · 11.11 kg/m (7.47 lb/ft)
- XXS — wall 11.07 mm · bore 38.2 mm · 13.44 kg/m (9.03 lb/ft)
From SCH 10 to XXS the pipe looks identical from outside, weighs 3.4 times as much, and its flow area drops from 2,358 to 1,146 mm² — less than half. Pump curves and pressure drops are calculated on the bore, so the schedule is not a detail.
NPS 4 (DN 100, OD 114.3 mm)
- SCH 10 — wall 3.05 mm · bore 108.2 mm · 8.37 kg/m (5.62 lb/ft)
- SCH 40 / STD — wall 6.02 mm · bore 102.3 mm · 16.07 kg/m (10.80 lb/ft)
- SCH 80 / XS — wall 8.56 mm · bore 97.2 mm · 22.32 kg/m (15.00 lb/ft)
- SCH 160 — wall 13.49 mm · bore 87.3 mm · 33.54 kg/m (22.54 lb/ft)
- XXS — wall 17.12 mm · bore 80.1 mm · 41.03 kg/m (27.57 lb/ft)
Where STD and schedule 40 stop being the same thing
At NPS 12 (OD 323.8 mm): STD is 9.53 mm and 73.86 kg/m, but schedule 40 is 10.31 mm and 79.70 kg/m — a separate row. XS is 12.70 mm and 97.43 kg/m. At NPS 10, schedule 60 and XS coincide at 12.70 mm (81.52 kg/m) while schedule 40 is 9.27 mm (60.29 kg/m). When a spec says "STD" on a large line, do not read it as schedule 40.
What the wall buys you
A thicker wall buys pressure and corrosion allowance. The Barlow estimate P = 2 × S × t / OD shows the scale: with an allowable stress of 138 MPa (A106 Grade B at ambient temperature), an NPS 2 schedule 40 pipe is good for roughly 2 × 138 × 3.91 / 60.3 = 17.9 MPa (179 bar), and schedule 80 for 25.4 MPa (254 bar) — before the mill tolerance, corrosion allowance and joint factors a piping code applies. Structural users care about the other side of the same number: for handrails, posts and frames the schedule is chosen for stiffness and weld thickness, and schedule 40 is the default because it is what every stockist holds.
How to choose
- Water, air, low-pressure process lines, structural tube: schedule 40 / STD. Cheapest, everywhere in stock, threads well down to NPS 2.
- Steam, hydraulics, threaded joints under pressure, anything with a corrosion allowance: schedule 80 / XS. Threading removes wall, and schedule 80 keeps enough.
- High pressure, thick-wall socket-weld systems: schedule 160 and XXS.
- Stainless and thin-wall lines: schedule 5S and 10S (the "S" series has slightly different walls for stainless).
- Sizes above NPS 10: write the wall thickness in millimeters on the drawing. STD, XS and the schedule numbers no longer say the same thing.
Tables
Every size from NPS 1/8 to 24 with every schedule, bore, weight per meter and per foot is in the pipe schedule chart; each size has its own page, for example NPS 2 / DN 50 and NPS 4 / DN 100. For European structural round tubes, which use a different size series, see the CHS table; for any other diameter and wall, the metal weight calculator.