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Steel Pipe Schedule Explained: Sch 40, Sch 80 and What It Means
Steel Pipe Schedule Explained: Sch 40, Sch 80 and What It Means
If you have ever received a quote for “2-inch pipe” and been quoted three wildly different wall thicknesses, you have met pipe schedule. Schedule (abbreviated “Sch”) is the number that defines the wall thickness of steel pipe for a given nominal size. Get it wrong and the pipe will either not thread onto your fittings, burst under pressure, or cost twice as much as it should.
What “Schedule” Actually Is
Schedule is not a wall thickness in millimeters. It is a dimensionless number derived from the formula: Schedule = 1000 × (P / S), where P is design pressure and S is allowable stress. In practice, it has become a table of standardized wall thicknesses. For a 2-inch pipe, Sch 40 has a wall around 3.91 milímetros; Sch 80 jumps to 5.54 milímetros. The outside diameter stays the same — only the wall gets thicker. That means the inside bore shrinks as schedule rises.
Common Schedules and Their Use
- Sch 10 / Sch 20: thin-wall, used for low-pressure conveyance and structural (bollards, railing). Cheapest, lightest.
- Sch 40: the default. Used for water, AVAC, low-pressure gas, general structural. This is what most buyers mean by “standard pipe”. Nosso Q235B welded pipe e GI round pipe are typically Sch 40 equivalent.
- Sch 80: heavier wall, used for medium-pressure process piping, steam, hydraulic and slurry. Same OD, smaller ID.
- Sch 160 / XXS: very heavy wall, for high pressure or corrosive service. Almost always seamless — see our 20# seamless pipe.
Nominal Bore vs Actual OD
This is the most confusing part. Pipe is sized by “nominal bore” (NPS), not actual OD. For NPS 1/8 through NPS 12, the outside diameter is fixed by schedule — NPS 2 is always 60.3 mm OD regardless of schedule. If you need pipe that slips over a roll or a spindle, specify OD, not NPS. If you need to match a fitting, specify NPS + schedule.
| NPS | DE (milímetros) | Sch 40 wall | Sch 80 wall |
|---|---|---|---|
| 1″ | 33.7 | 3.38 milímetros | 4.55 milímetros |
| 2″ | 60.3 | 3.91 milímetros | 5.54 milímetros |
| 4″ | 114.3 | 6.02 milímetros | 8.56 milímetros |
| 6″ | 168.3 | 7.11 milímetros | 10.97 milímetros |
How to Specify Pipe Correctly
Send: material grade (Q235B / Q355B / 20# / 304 / 316eu), NPS, schedule, length (6 m standard), end finish (plain / beveled / threaded) and standard (ASTM A53 / A106 / API 5L / GB). Stainless pipe buyers should also specify finish (2B / pickled / bright annealed) — see our 304 welded pipe range.
