Pipe schedule is one of the most commonly misunderstood concepts in plant engineering. The schedule number is not a direct measurement of wall thickness — it is a pressure rating designation that relates to the allowable stress of the pipe material. Two pipes of the same nominal size but different schedules have the same outside diameter but different wall thicknesses, inside diameters, and pressure ratings.
Understanding pipe schedules, nominal pipe size (NPS), and the relationship between wall thickness and pressure class is fundamental knowledge for anyone working with process piping, power piping, or structural tubing. Specifying the wrong schedule can result in a pipe that fails under operating pressure or one that is unnecessarily heavy and expensive.
This guide explains the NPS system, schedule number origins and meaning, how to look up wall thickness for any size and schedule, and the material grade standards that determine allowable pressure.
Nominal Pipe Size and Outside Diameter
Nominal Pipe Size (NPS) is a dimensionless designator — it does not correspond directly to any physical measurement on the pipe. For NPS 1/8 through NPS 12, the outside diameter is fixed at a value larger than the nominal size. For example, NPS 2 pipe has an outside diameter of 2.375 inches, not 2.000 inches. NPS 4 has an OD of 4.500 inches. This historical quirk dates back to the original iron pipe standards where NPS referred approximately to the inside diameter.
For NPS 14 and above, the outside diameter equals the nominal size. NPS 14 has an OD of 14.000 inches, NPS 16 has an OD of 16.000 inches, and so on. This transition creates a discontinuity that catches people off guard when working with larger pipe sizes.
The critical point is that the outside diameter is constant for a given NPS, regardless of schedule. All NPS 4 pipe — whether Schedule 10, 40, 80, 160, or XXH — has the same 4.500-inch OD. Only the wall thickness (and therefore the inside diameter and weight) changes between schedules.
Common NPS sizes in plant piping are 1/2, 3/4, 1, 1-1/2, 2, 3, 4, 6, 8, 10, 12, 14, 16, 18, 20, and 24. Sizes above 24 NPS are used in large water mains, penstocks, and refinery process lines. Below NPS 1/2, small-bore tubing (measured by actual OD and wall thickness, not NPS) is typically used instead of pipe.
Schedule Numbers and Their Origin
The original schedule number formula was: Schedule = 1000 x P / S, where P is the internal pressure in PSI and S is the allowable stress in PSI. Schedule 40 was designed for a working pressure of approximately 40/1000 of the allowable stress. This formula is no longer used directly for design, but the schedule designations it produced remain the standard for specifying pipe wall thickness.
Standard schedule numbers defined in ASME B36.10 (carbon and alloy steel) are: 5, 10, 20, 30, 40, 60, 80, 100, 120, 140, and 160. Additionally, three legacy designations remain in common use: STD (Standard), XH or XS (Extra Heavy/Extra Strong), and XXH or XXS (Double Extra Heavy/Double Extra Strong).
For NPS 1/8 through NPS 10, Schedule 40 and STD are the same wall thickness. Schedule 80 and XH are the same. Above NPS 10, the equivalencies diverge — STD wall thickness no longer matches Schedule 40. This is a frequent source of error. Always verify the actual wall thickness from the dimension tables rather than assuming STD equals Schedule 40.
Common wall thickness examples for NPS 4 (OD = 4.500 inches): Schedule 10 = 0.120 inches wall (ID = 4.260), Schedule 40/STD = 0.237 inches (ID = 4.026), Schedule 80/XH = 0.337 inches (ID = 3.826), Schedule 160 = 0.531 inches (ID = 3.438), XXH = 0.674 inches (ID = 3.152). Each step up in schedule adds wall thickness, reduces ID, increases weight, and raises the pressure rating.
Pipe Material Grades and Standards
The pipe material grade determines the allowable stress, which in turn determines the pressure rating at a given schedule and temperature. The most common carbon steel pipe specification is ASTM A106 Grade B (seamless) for high-temperature service and ASTM A53 Grade B (welded or seamless) for general service. Both have a minimum yield strength of 35,000 PSI and a minimum tensile strength of 60,000 PSI.
For low-temperature service (below -20°F), ASTM A333 Grade 6 is specified because it has guaranteed impact toughness at low temperatures. For high-temperature, high-pressure service in power plants and refineries, alloy steel grades like ASTM A335 (chrome-moly) are used. Common alloy grades include P11 (1-1/4 Cr - 1/2 Mo), P22 (2-1/4 Cr - 1 Mo), and P91 (9 Cr - 1 Mo - V), each with increasing temperature capability.
Stainless steel pipe follows ASTM A312 for welded and seamless austenitic grades (304, 316, 321, 347) and ASTM A790 for duplex grades (2205, 2507). Stainless grades are designated by their UNS number or common name: TP304 (18 Cr - 8 Ni), TP316 (18 Cr - 10 Ni - 2 Mo), TP304L and TP316L (low-carbon variants for welding). Stainless pipe uses the schedule system defined in ASME B36.19, which includes schedules 5S, 10S, 40S, and 80S. The 'S' suffix indicates stainless schedule, which may differ from the carbon steel schedules at some sizes.
When ordering pipe, the complete specification includes: NPS, schedule, material grade, product form (seamless or welded), and any additional requirements (hydrostatic test, NDE, impact testing). For example: NPS 6, Schedule 40, ASTM A106 Grade B, Seamless. Omitting any element invites errors in procurement and fabrication.
Calculating Allowable Pressure
The allowable internal pressure for a straight pipe is calculated per ASME B31.1 (power piping) or B31.3 (process piping): P = 2 x S x E x t / (D - 2 x Y x t), where P is the maximum allowable pressure in PSI, S is the allowable stress at design temperature (from the ASME code tables), E is the joint efficiency (1.0 for seamless, 0.85 for ERW, 0.80 for furnace butt-weld), t is the minimum wall thickness in inches (nominal minus mill tolerance and corrosion allowance), D is the outside diameter in inches, and Y is a coefficient based on material and temperature (typically 0.4 for carbon steel below 900°F).
Mill tolerance for seamless pipe is -12.5 percent of nominal wall thickness. This means a Schedule 40 NPS 4 pipe with a nominal wall of 0.237 inches has a minimum wall of 0.237 x 0.875 = 0.207 inches. Corrosion allowance — typically 1/16 inch (0.0625 inches) for carbon steel in clean service — is subtracted from the minimum wall to arrive at the thickness used in the pressure calculation.
For NPS 4, Schedule 40, A106 Grade B seamless pipe at 650°F (S = 15,000 PSI from ASME B31.1): t_min = 0.237 x 0.875 - 0.0625 = 0.145 inches. P = 2 x 15,000 x 1.0 x 0.145 / (4.500 - 2 x 0.4 x 0.145) = 4,350 / 4.384 = 992 PSI. This is the maximum allowable working pressure at 650°F.
At ambient temperature (S = 20,000 PSI for A106 Gr B), the allowable pressure increases proportionally. This is why hydrostatic tests are conducted at 1.5 times design pressure at ambient temperature — the higher allowable stress at low temperature provides the margin for the test.
Practical Pipe Schedule Selection
For general plant services — compressed air, cooling water, low-pressure steam — Schedule 40 carbon steel is the default selection. It handles pressures up to several hundred PSI depending on size and temperature, it is universally stocked, and fitters and welders are thoroughly familiar with it. Unless the design conditions demand otherwise, start with Schedule 40.
Schedule 80 is specified when higher pressures are needed, when threaded connections are used (the thicker wall provides more thread engagement), or when corrosion or erosion will reduce wall thickness over the service life. Schedule 80 pipe weighs roughly 40 to 50 percent more than Schedule 40 and costs proportionally more per foot.
Schedule 10 and 10S are common choices for stainless steel piping in low-pressure applications (drains, vents, food-grade systems) where corrosion is not a concern and the higher material cost of stainless makes wall thickness savings significant. Schedule 10S stainless weighs roughly half as much per foot as Schedule 40S.
For high-pressure applications (boiler piping, high-pressure steam, hydraulic systems), Schedules 120, 160, and XXH are used. These heavy-wall pipes require special welding procedures, longer preheat and post-weld heat treatment cycles, and heavier support structures. Always verify that the flanges, fittings, and valves specified in the system are rated to match the pipe pressure class — the system is only as strong as its weakest component.