What Does Schedule 40 Mean?
Schedule 40, written as Sch 40, is a wall-thickness series designation for steel pipe defined in ASME B36.10. The schedule number is not a direct measurement in millimetres; it is a series index derived from the design formula in which the schedule number approximates 1000 times the design pressure divided by the allowable stress. As the schedule number rises, the wall thickness increases and the pipe can carry higher pressure for the same outside diameter. Schedule 40 is the most widely stocked series because it balances strength, weight and cost for general industrial service.
Sch 40 Wall Thickness by NPS
For each nominal pipe size, Sch 40 has a fixed wall thickness. Common values are:
| NPS | Outside Diameter (mm) | Sch 40 Wall (mm) |
|---|---|---|
| 1/2 | 21.3 | 2.77 |
| 3/4 | 26.7 | 2.87 |
| 1 | 33.4 | 3.38 |
| 1-1/2 | 48.3 | 3.68 |
| 2 | 60.3 | 3.91 |
| 3 | 88.9 | 5.49 |
| 4 | 114.3 | 6.02 |
| 6 | 168.3 | 7.11 |
| 8 | 219.1 | 8.18 |
| 10 | 273.0 | 9.27 |
| 12 | 323.8 | 10.31 |
Seamless pipe in Sch 40 is produced by hot piercing and rolling, then sized to the exact outside diameter and wall thickness of this table.
Sch 40 vs Sch 40S
For stainless steel pipe, the wall series is defined separately in ASME B36.19, where the "S" suffix distinguishes the stainless schedule. For many sizes up to NPS 12, Sch 40S equals Sch 40 in wall thickness, but at larger sizes the two series diverge. This means a carbon steel Sch 40 pipe and a stainless steel Sch 40S pipe of the same NPS may have different wall thicknesses at large diameters, so the correct series must be stated in the order.
Why Sch 40 Is So Widely Used
Pressure capability: the thicker wall of Sch 40 carries substantially higher internal pressure than Sch 10 or Sch 20 for the same diameter.
Mechanical strength: the heavier wall resists bending loads, vibration and external impact better than thin-wall pipe.
Threading: Sch 40 is the minimum wall commonly recommended for cutting NPT threads without weakening the pipe end.
Availability: Sch 40 seamless pipe is stocked worldwide in carbon steel and stainless grades, which shortens project lead times.
Typical Application Scenarios
Process piping in refineries and chemical plants for water, steam and general fluids.
Fire-protection and sprinkler systems, where the wall must survive handling and hydrostatic testing.
Compressed air and utility lines in industrial plants.
Structural and mechanical applications where the pipe also carries fluid.
Selection and Ordering Points
Confirm the wall thickness value for the exact NPS, because the same schedule number gives different millimetre values at different sizes.
State the product standard (for example, ASTM A106 or A53 for carbon steel) together with the schedule, since the schedule alone does not define the material.
For stainless pipe, specify 40S per ASME B36.19 so the correct series is used at large diameters.
Check the tolerance class: standard and special tolerances exist for outside diameter and wall thickness.
FAQ
Q1: Does Sch 40 mean a 40 mm wall thickness?
A: No. Sch 40 is a series designation, and the actual wall thickness depends on the pipe size, from 2.77 mm at NPS 1/2 up to 10.31 mm at NPS 12.
Q2: What is the difference between Sch 40 and Sch 40S?
A: Sch 40 is the carbon steel series in ASME B36.10; Sch 40S is the stainless steel series in ASME B36.19. They are equal at many sizes but diverge at larger NPS.
Q3: Can Sch 40 pipe be threaded?
A: Yes, Sch 40 is the common minimum wall for threading; thinner walls such as Sch 10 do not leave enough material for strong threads.
Q4: How do I choose between Sch 40 and Sch 80?
A: Choose Sch 80 when the design pressure or external load requires a thicker wall; otherwise Sch 40 offers lower weight and cost for the same diameter.
Q5: Is schedule 40 available in seamless form?
A: Yes, Sch 40 seamless pipe is produced in carbon steel and stainless grades from NPS 1/8 up to NPS 24 or larger, depending on the mill.
Q6: How is the schedule number calculated?
A: The schedule number approximates 1000 times the design pressure divided by the allowable stress, which is why higher numbers mean thicker walls.