Pipe Schedule Selection —
When to Use Sch 40,
80 or 160

The schedule number printed on a length of pipe is one of the most frequently misunderstood pieces of information in process pipework design. Engineers specify Sch 40 because that is what was used last time, or Sch 80 because it feels safer, without understanding what those numbers actually represent or how they relate to the pressure the pipe will carry.

This article explains the pipe schedule system from first principles — where it came from, what it means, how to select the correct schedule for a given application, and the common errors that result from applying it without understanding it.

Where Schedule Numbers Come From

The pipe schedule system originated in the early twentieth century as a standardised way of specifying pipe wall thickness. Prior to standardisation, pipe manufacturers used their own wall thickness designations — Standard Weight (STD), Extra Strong (XS) and Double Extra Strong (XXS) — which gave no direct indication of pressure capacity and varied between manufacturers.

The American Standards Association introduced the schedule number system in the 1930s, later formalised in ASME B36.10 for carbon and alloy steel pipe, and ASME B36.19 for stainless steel pipe. The schedule numbers — 5, 10, 20, 30, 40, 60, 80, 100, 120, 140, 160 — are index numbers, not wall thicknesses. The actual wall thickness corresponding to a given schedule number varies with the nominal pipe size (NPS).

How Schedule Numbers Relate to Wall Thickness

The relationship between schedule number, nominal pipe size and wall thickness is defined in ASME B36.10M. The formula behind the schedule numbering system is:

Schedule Number ≈ 1000 × (P / S)

Where P is the internal design pressure in psi, and S is the allowable stress of the pipe material in psi at the design temperature. This formula shows the intent of the system — a higher schedule number reflects either a higher design pressure, a lower material allowable stress, or both. It also shows immediately why schedule alone is insufficient to specify a pipe for pressure service: the same schedule number at different sizes produces different wall thicknesses.

Some representative wall thicknesses from ASME B36.10M to illustrate the point:

The wall thickness increases with both schedule number and nominal size. At NPS 12, Sch 160 wall is 33.32mm — nearly as thick as the pipe's bore. At NPS ½", Sch 160 wall is 4.78mm — the bore is under 12mm. The schedule system spans an enormous range of pressure capacity within a single designation.

Standard Weight, Extra Strong and XXS

The legacy designations — STD, XS and XXS — predate the schedule system and remain in common use, particularly in older specifications, oil and gas applications, and general industrial pipework. Their relationship to schedule numbers is:

ASME B36.10 vs ASME B36.19

Two separate ASME standards govern pipe dimensions:

ASME B36.10M covers welded and seamless wrought steel pipe in carbon steel, alloy steel, and other non-stainless materials. Schedules range from 5 through to 160 and XXS across the full NPS range from ⅛" to 24" and beyond.

ASME B36.19M covers stainless steel pipe. It uses the same OD as B36.10 but introduces S-suffix schedules — 5S, 10S, 40S and 80S — which at some sizes differ from the equivalent B36.10 schedule. The S-suffix schedules were introduced partly because stainless steel's higher corrosion resistance allows thinner walls to be used compared to carbon steel in many applications, and partly to reflect stainless steel's different pressure-temperature performance characteristics.

For stainless pipework, always reference B36.19. Specifying "Sch 40" on a stainless pipe order without the S suffix introduces ambiguity — confirm whether Sch 40S is intended or the full B36.10 Sch 40 wall (which may be heavier at some sizes).

Pressure Capacity — The Barlow Formula

The pressure capacity of a pipe is calculated using the Barlow formula (or the more complete Lamé equation for thick-walled pipe, but Barlow is used for the vast majority of standard process pipework):

P = (2 × S × t) / OD

Where P is the internal pressure (MPa or psi), S is the allowable hoop stress of the material at temperature (MPa or psi), t is the wall thickness (mm or inches), and OD is the outside diameter (mm or inches).

Rearranging to find the minimum required wall thickness:

t = (P × OD) / (2 × S)

This is the fundamental relationship that should drive schedule selection — not convention, not what was used last time, and not a round number chosen for comfort. A pipe schedule is correct when its wall thickness meets or exceeds the calculated minimum required thickness, with any applicable corrosion allowance, mill tolerance and manufacturing factor applied on top.

Allowable Stress Values

The allowable stress S is not fixed — it is a function of both material grade and operating temperature, taken from the governing code's stress tables. For carbon steel A106 Grade B at ambient temperature, the allowable stress per ASME B31.3 is approximately 137.9 MPa (20,000 psi). At 400°C, it drops to approximately 82.7 MPa. This temperature dependency is why the same pipe at elevated temperature cannot hold the same pressure as at ambient — the material is weaker, and the wall thickness (or schedule) must be increased to compensate.

Worked Example — Schedule Selection

To make this concrete: a carbon steel process line (A106 Gr.B) at NPS 4, design pressure 50 bar (5.0 MPa), design temperature 200°C.

From ASME B31.3 stress tables: S for A106 Gr.B at 200°C ≈ 130 MPa.

OD of NPS 4 = 114.3mm.

Minimum wall thickness: t = (5.0 × 114.3) / (2 × 130) = 2.20mm

Now apply corrections:

• Corrosion allowance: 1.5mm (typical for mildly corrosive carbon steel)
• Mill tolerance: ASME B36.10 allows −12.5% on wall thickness, so divide by 0.875

Corrected minimum: (2.20 + 1.5) / 0.875 = 4.23mm nominal wall required

Checking against the schedule table for NPS 4:

• Sch 40: 6.02mm — adequate (4.23mm required, 6.02mm provided)
• Sch 10: 3.05mm — not adequate

Sch 40 is the correct selection for this application. There is no engineering justification for Sch 80 (8.56mm) or Sch 160 (13.49mm) unless additional corrosion allowance, erosion allowance, or a higher pressure scenario is being provided for.

When Sch 40 Is Not Enough

Sch 40 is the default workhorse of process pipework. The question is not "why would I use Sch 80?" — it is "what specific conditions require a heavier schedule?"

High Pressure

At high design pressures the calculated minimum wall thickness will exceed Sch 40, driving the selection to Sch 80, Sch 120, Sch 160 or XXS. High-pressure utility systems (steam supply, high-pressure water, hydraulics), wellhead pipework, and high-pressure chemical systems all commonly require schedules above 40.

Elevated Temperature

As shown in the allowable stress table above, material strength reduces at elevated temperature. A carbon steel system that is comfortably within Sch 40 at ambient may require Sch 80 at design temperature once the reduced allowable stress is applied. Always calculate at design temperature, not ambient — this is a common source of under-specification.

Corrosion and Erosion Allowance

If the process fluid is corrosive or erosive, a corrosion allowance is added to the calculated minimum wall to ensure the pipe retains adequate structural integrity at end of design life. On carbon steel pipe in moderate corrosive service, a 1.5–3.0mm corrosion allowance is common. On highly corrosive service or where erosion from entrained solids is expected, the allowance may be 3–6mm or more. This added wall pushes many lines from Sch 40 into Sch 80 territory.

Threaded Connections

On screwed (NPT or BSP threaded) connections at smaller pipe sizes, the threading operation removes material from the outer surface of the pipe wall. ASME B31.3 and the relevant pipe standards require a minimum remaining wall after threading. At NPS 1" and below, Sch 40 is frequently the minimum schedule that provides adequate wall after threading — at smaller sizes, Sch 80 is often mandated for threaded end connections.

Mechanical Loading

In applications where the pipe carries significant external load — buried pipe, pipe supporting its own weight over long unsupported spans, or pipe subject to vibration — the wall thickness may be determined by structural requirements rather than pressure. In these cases the minimum schedule is set by deflection or stress calculations rather than the Barlow formula.

Vacuum Service

Under external pressure (vacuum service), the critical failure mode is collapse rather than burst. Thinner-walled pipe is more susceptible to collapse buckling. External pressure calculations per ASME are considerably more complex than internal pressure, and frequently drive heavier schedules than internal pressure calculations alone would indicate.

When Sch 80, 120, 160 Are Appropriate

The S-Suffix Schedules for Stainless Steel

For stainless pipework to ASME B36.19, the common schedules are 5S, 10S, 40S and 80S. At some pipe sizes these are thinner than their B36.10 equivalents — reflecting the fact that stainless steel often carries lower design pressures (water, chemical, food processing) and the corrosion-resistant nature of the material means a corrosion allowance is often zero or minimal, allowing a lighter wall.

Sch 10S is the most common schedule for stainless process lines in food, dairy, pharmaceutical and cleanroom applications — it is adequately strong for typical pressures, easier to weld due to lower wall thickness, and costs less in material than Sch 40S.

Sch 40S should be specified where pressure or mechanical loading requires it, not as a default. For high-purity applications with orbital welding, the reduced wall of Sch 10S also produces a smaller, more consistent internal bead which is preferred from a cleanability standpoint.

Manufacturing Tolerance and Its Impact

ASME B36.10 permits a wall thickness tolerance of −12.5% on the nominal wall. This means a pipe ordered as Sch 40 NPS 4 with a nominal wall of 6.02mm may be supplied with an actual wall as low as 5.27mm — and this is fully within specification.

This tolerance must be accounted for in pressure calculations. The correct approach is to work with the minimum wall (nominal minus 12.5%) when calculating available pressure capacity, or equivalently, to divide the calculated minimum wall by 0.875 when determining the required nominal wall — as demonstrated in the worked example above.

Failing to account for mill tolerance is a common calculation error that results in pipes being specified at a nominal wall that does not actually meet the pressure requirement once tolerance is considered.

Common Mistakes in Schedule Specification

1. Specifying Sch 40 as a default without calculation. On low-pressure, large-bore, corrosive service, or elevated temperature lines, Sch 40 may not meet the minimum wall requirement once corrosion allowance and mill tolerance are applied.
2. Over-specifying on low-pressure lines. Sch 80 or Sch 160 on a 10 bar utility water line is unnecessary weight and unnecessary cost — particularly at larger diameters where the weight penalty is significant and affects support sizing.
3. Ignoring temperature when selecting schedule. Calculating at ambient temperature and then operating at elevated temperature gives unconservative results. Design temperature is the governing condition.
4. Not accounting for mill tolerance. Using nominal wall in the pressure calculation without the −12.5% tolerance gives an unconservative result.
5. Mixing B36.10 and B36.19 schedules. Specifying "Sch 40" on stainless pipe without clarifying whether B36.10 or B36.19 (Sch 40S) is intended creates ambiguity. At some sizes the wall thicknesses differ.
6. Applying the same schedule to all lines in a system. Process systems commonly contain lines at significantly different pressures and temperatures — instrument impulse lines, drain connections and main process headers may all be different nominal sizes and pressures. Each line should be individually assessed.
7. Specifying Sch 10 on threaded end connections. Thin-walled pipe cannot always be threaded to full engagement depth. Check minimum wall requirements for threading before specifying Sch 5 or Sch 10 on threaded lines.

Summary

Pipe schedule selection is a calculation, not a convention. The correct schedule is the lightest schedule whose nominal wall thickness, after applying mill tolerance and all relevant allowances, meets the calculated minimum required wall under the governing design conditions — which means design pressure and design temperature, not ambient.

Sch 40 is correct for a large proportion of general process pipework. Sch 80 is appropriate where pressure, temperature, corrosion allowance or mechanical loading drives the wall requirement above what Sch 40 provides. Sch 120, 160 and XXS serve specific high-pressure applications. Sch 10 and 5S serve low-pressure, non-corrosive service and are standard for stainless cleanroom and food process lines.

Over-specification wastes money, adds weight, and increases support requirements. Under-specification is a safety issue. Do the calculation.