Pipe Schedule Chart: NPS, DN, Wall Thickness & Sizes Explained

Quick Answer

Pipe size is defined by two independent values: NPS (Nominal Pipe Size) or its metric equivalent DN (Diamètre Nominal), which fixes the outside diameter; and the schedule number (SCH), which fixes the wall thickness. The crucial point is that for a given NPS, the outside diameter never changes — increasing the schedule thickens the wall inward, reducing the bore. This is what allows fittings, flanges, and valves to fit any schedule of the same NPS. NPS is a dimensionless designator, not a measurement: NPS 2 pipe has an OD of 60.3 mm (2.375"), not 2 inches. From NPS 14 and above the OD equals the nominal size in inches (NPS 14 = 355.6 mm = 14.000"). Common schedules run SCH 5, 10, 20, 30, 40, 60, 80, 100, 120, 140, 160, plus the older designations STD (Standard), XS (Extra Strong) and XXS (Double Extra Strong). STD matches SCH 40 up to NPS 10, and XS matches SCH 80 up to NPS 8; beyond those sizes STD is fixed at 9.53 mm and XS at 12.7 mm regardless of diameter. Dimensions are governed by ASME B36.10M for welded and seamless wrought steel pipe and ASME B36.19M for stainless steel pipe, where schedules carry an "S" suffix (10S, 40S, 80S).

Pipe sizing confuses newcomers for a specific, avoidable reason: the number in the size does not measure anything. A "2-inch pipe" has neither a 2-inch outside diameter nor a 2-inch bore — its OD is 2.375 inches (60.3 mm), and its inside diameter depends entirely on which schedule you specify. Order 2-inch SCH 40 and you get a 52.5 mm bore; order 2-inch SCH 160 and you get 38.1 mm through the same 60.3 mm outside diameter.

Once you see the logic, it is elegant. The outside diameter is fixed by NPS so that flanges, fittings, valves, supports, and welding equipment all fit regardless of wall thickness. The wall thickness is set by schedule to suit the pressure, temperature, and corrosion allowance the line needs. Two independent variables, one standardised outside dimension — that is what makes industrial piping interchangeable across manufacturers and countries.

Getting it wrong is expensive. Order the right NPS and wrong schedule and the pipe arrives with a bore that will not pass design flow, or a wall too thin for the design pressure. Assume STD equals SCH 40 at NPS 12 and your wall thickness is out by nearly a millimetre. Specify a schedule for stainless without the "S" suffix and you may receive a heavier, costlier wall than intended.

For piping designers, procurement teams, fabricators, and anyone building a material take-off — this guide explains NPS and DN, how schedules work, the reference dimensions, and how to specify pipe correctly.

For material selection, see Carbon Steel Pipe, Stainless Steel Pipe, API 5L Line Pipe, and Alloy Steel Pipe A335.

NPS and DN

NPS (Nominal Pipe Size) is the North American designator, dimensionless, based on inches. DN (Diamètre Nominal) is the ISO/European equivalent, based loosely on millimetres. They are conventional equivalents, not conversions.

The key break: below NPS 14, the OD bears no simple relationship to the nominal number. From NPS 14 upward, OD equals the nominal size in inches exactly.

How Schedules Work

The schedule number sets wall thickness. It derives historically from an approximation of the ratio of internal pressure to allowable stress — higher schedule, thicker wall, higher pressure capability.

The essential rule: increasing the schedule does not change the OD. The extra metal goes inward, reducing the bore.

Worked example, NPS 2 (OD 60.3 mm):

Same OD throughout — same flanges, same fittings, same pipe supports. Only the bore changes.

STD, XS and XXS

These older designations predate the schedule system and remain in daily use.

• STD (Standard) — equals SCH 40 through NPS 10. From NPS 12 upward, STD is fixed at 9.53 mm (0.375") regardless of diameter.
• XS (Extra Strong) — equals SCH 80 through NPS 8. From NPS 10 upward, XS is fixed at 12.70 mm (0.500").
• XXS (Double Extra Strong) — a separate heavy series, generally thicker than SCH 160 in smaller sizes but not consistently related to any schedule.

This is a classic error source. At NPS 12, STD (9.53 mm) is not SCH 40 (10.31 mm). At NPS 10, XS (12.70 mm) is not SCH 80 (15.09 mm). Never assume the equivalence holds in large sizes.

Common Schedule Reference (Carbon Steel, ASME B36.10M)

Wall thickness in millimetres:

Reference values for planning. For design, fabrication, and purchase, always verify against the current edition of ASME B36.10M — dimensions and available schedules vary by size and are periodically revised.

ASME B36.10M vs B36.19M

ASME B36.10M — Welded and Seamless Wrought Steel Pipe. Covers carbon steel and alloy steel. Uses plain schedule numbers: SCH 10, 40, 80, 160, plus STD/XS/XXS.

ASME B36.19M — Stainless Steel Pipe. Uses schedules with an "S" suffix: 5S, 10S, 20S, 40S, 80S.

The S-schedules are generally thinner than the corresponding carbon steel schedules in larger sizes, reflecting stainless steel's higher allowable stress and the fact that it needs less corrosion allowance. In small sizes 40S and SCH 40 often coincide, which is why the distinction is easy to miss.

Practical consequence: specifying "SCH 40 stainless" when you mean "40S" (or vice versa) can bring the wrong wall thickness and a significant cost difference. State the standard explicitly.

For stainless pipe grades, see Stainless Steel Pipe.

Choosing a Schedule

Schedule selection follows from the pressure design calculation, not from habit:

1. Design pressure and temperature — determines required thickness via the piping code formula (ASME B31.3 for process, B31.1 for power, B31.4/B31.8 for pipelines)
2. Material allowable stress at design temperature
3. Corrosion allowance — added to the calculated pressure-design thickness, typically 1.5–3 mm for carbon steel in corrosive service
4. Mill tolerance — seamless pipe carries a −12.5% wall tolerance, which must be accounted for
5. Mechanical and handling considerations — small-bore lines are often thickened beyond pressure requirements for robustness
6. Erosion/velocity — high-velocity or erosive service may demand extra wall
7. Threading — threaded connections require a minimum wall (SCH 80 is commonly the minimum for threaded carbon steel)

Then round up to the next commercially available schedule.

Typical Usage

Related Dimensions

Fittings and flanges are specified by NPS and matched to the pipe schedule bore. Butt-weld fittings per ASME B16.9 are ordered with a schedule so the bores align — a mismatched bore creates a weld-root step, turbulence, and a stress raiser. See Pipe Fittings.

Weight is commonly quoted in kg/m or lb/ft and scales with wall thickness; it drives transport, support spacing, and structural loading.

Length is usually supplied as single random (roughly 5–7 m) or double random (roughly 11–13 m), or cut to specified lengths.

Seamless vs welded is independent of schedule — both are available across most schedules. See Seamless vs Welded Pipe.

Common Specification Mistakes

After 15+ years supplying piping materials to industrial projects:

Mistake 1: Assuming NPS Equals a Real Dimension

Designer assumes NPS 2 pipe has a 2-inch bore or OD, and sizes flow accordingly.

Prevention: NPS is a designator. Use the actual OD from the table and calculate ID from OD minus twice the wall thickness for the specified schedule.

Mistake 2: Assuming STD = SCH 40 in Large Sizes

STD ordered for NPS 12 expecting SCH 40. STD is 9.53 mm; SCH 40 is 10.31 mm — the pipe is thinner than the design assumed.

Prevention: Remember STD = SCH 40 only through NPS 10, and XS = SCH 80 only through NPS 8. Above those, state the schedule explicitly.

Mistake 3: Confusing S-Schedules with Carbon Schedules

"SCH 40 stainless" ordered when 40S was intended, or vice versa. Wall thickness and cost differ.

Prevention: For stainless, specify per ASME B36.19M with the S suffix (10S, 40S, 80S). For carbon and alloy, specify per B36.10M.

Mistake 4: Ignoring Mill Tolerance

Pressure design done at nominal wall thickness, ignoring the −12.5% seamless tolerance. Actual minimum wall is below the code requirement.

Prevention: Apply the mill tolerance in the pressure design calculation, or specify minimum wall pipe.

Mistake 5: Forgetting Corrosion Allowance

Wall calculated for pressure only. After several years of corrosion the pipe falls below required thickness.

Prevention: Add the project corrosion allowance to the pressure-design thickness before selecting the schedule.

Mistake 6: Mismatched Fitting Bore

Fittings ordered by NPS without specifying schedule. Bores do not match the pipe, creating a root step at the weld.

Prevention: Order butt-weld fittings to the same schedule as the pipe, and state it on the requisition.

Mistake 7: Threading Thin-Wall Pipe

SCH 40 pipe threaded where SCH 80 is required. Thread cutting removes too much wall and the pipe fails at the thread root.

Prevention: Use SCH 80 or heavier for threaded connections in carbon steel, and check the code minimum for the service.

Supply from Kasko Makine

Kasko Makine supplies pipe in all standard schedules and sizes for oil & gas, petrochemical, power, water, and industrial projects:

Pipe types:

• Carbon steel pipe (ASTM A106 Gr B, A53, A333 low temperature)
• Line pipe (API 5L PSL1/PSL2, grades B through X70)
• Alloy steel pipe (ASTM A335 P11/P22/P91 chrome-moly)
• Stainless steel pipe (ASTM A312 TP304/304L/316/316L/321)
• Duplex and super duplex
• Seamless and welded (ERW, LSAW, SSAW)

Sizes and schedules:

• NPS ⅛" through 60"
• SCH 5, 10, 20, 30, 40, 60, 80, 100, 120, 140, 160
• STD, XS, XXS
• Stainless schedules 5S, 10S, 20S, 40S, 80S
• Dimensions per ASME B36.10M and B36.19M

Ends: plain end, bevelled end, threaded, with couplings

Certification: EN 10204 Type 3.1/3.2 material test certificates, dimensional and wall thickness verification, hydrostatic and NDE reports, PMI, heat traceability, NACE MR0175 where required

Engineering support:

• Schedule selection against design pressure and temperature
• Corrosion allowance and mill tolerance review
• Matching fitting and flange bore specification
• Material take-off (MTO) support and cross-checking
• Weight and transport calculation

Logistics: Pipe shipped from Istanbul to projects across Africa, the Middle East, Central Asia, and beyond. Standard carbon steel 4-8 weeks; alloy and stainless 8-14 weeks; large diameter and specialty grades by project schedule.

Need pipe in a specific schedule? Send us the NPS or DN, schedule, material grade and specification, end preparation, quantity and required certification to info@kaskomakine.com or WhatsApp +90 (537) 521 1399. We'll confirm dimensions against ASME B36.10M/B36.19M, check availability, and provide pricing within 48 hours.

Continue Reading: Pipe Guides

• Carbon Steel Pipe: Grades, Sizes & Standards — Carbon steel specifications
• Stainless Steel Pipe — Stainless grades and B36.19M schedules
• API 5L Line Pipe — Oil and gas line pipe
• Alloy Steel Pipe A335 — Chrome-moly high-temperature pipe
• Seamless vs Welded Pipe — Manufacturing method comparison
• Pipe Fittings — Matching fitting bores

Frequently Asked Questions

Q: What is a pipe schedule?
A: A pipe schedule is a number that defines the wall thickness of a pipe for a given nominal size. Pipe size is set by two independent values: NPS (Nominal Pipe Size) or DN, which fixes the outside diameter, and the schedule number, which fixes the wall thickness. The critical principle is that for a given NPS the outside diameter never changes — increasing the schedule adds metal inward, thickening the wall and reducing the bore. This keeps the outside dimension constant so flanges, fittings, valves, and supports fit any schedule of the same size. Common schedules are SCH 5, 10, 20, 30, 40, 60, 80, 100, 120, 140, and 160, along with the older designations STD (Standard), XS (Extra Strong), and XXS (Double Extra Strong).

Q: Is NPS the same as the pipe's actual diameter?
A: No. NPS (Nominal Pipe Size) is a dimensionless designator, not a measurement. NPS 2 pipe has an outside diameter of 60.3 mm (2.375 inches), not 2 inches, and its inside diameter varies with schedule — 52.5 mm in SCH 40 but only 38.2 mm in XXS. Below NPS 14, the nominal number bears no simple relationship to any actual dimension. From NPS 14 upward, however, the outside diameter equals the nominal size in inches exactly: NPS 14 is 355.6 mm (14.000 inches), NPS 24 is 610 mm (24.000 inches). Always take the OD from a dimensional table and calculate the ID as OD minus twice the wall thickness for the specified schedule.

Q: What is the difference between SCH 40 and SCH 80?
A: SCH 40 and SCH 80 differ in wall thickness, not outside diameter. For NPS 2 pipe with a 60.3 mm OD, SCH 40 has a 3.91 mm wall giving a 52.5 mm bore, while SCH 80 has a 5.54 mm wall giving a 49.2 mm bore. SCH 80 therefore withstands higher pressure and offers greater mechanical strength and corrosion margin, at higher weight and cost with reduced flow area. SCH 40 (equivalent to STD up to NPS 10) is the general-purpose workhorse for process and utility piping. SCH 80 (equivalent to XS up to NPS 8) is specified for higher pressures, threaded connections where thread cutting removes wall material, and services needing extra mechanical robustness.

Q: What is the difference between STD, XS and XXS?
A: STD (Standard), XS (Extra Strong), and XXS (Double Extra Strong) are older wall-thickness designations that predate the schedule system and remain in common use. STD equals SCH 40 through NPS 10, but from NPS 12 upward it is fixed at 9.53 mm (0.375 inches) regardless of diameter. XS equals SCH 80 through NPS 8, but from NPS 10 upward it is fixed at 12.70 mm (0.500 inches). XXS is a separate heavy series that is generally thicker than SCH 160 in smaller sizes but has no consistent relationship to any schedule. The fixed thicknesses in larger sizes are a common source of specification error — never assume STD equals SCH 40 above NPS 10.

Q: What is the difference between ASME B36.10M and B36.19M?
A: ASME B36.10M covers welded and seamless wrought steel pipe — carbon steel and alloy steel — using plain schedule numbers such as SCH 10, 40, 80, and 160, plus STD, XS, and XXS. ASME B36.19M covers stainless steel pipe and uses schedules with an "S" suffix: 5S, 10S, 20S, 40S, and 80S. The S-schedules are generally thinner than the corresponding carbon steel schedules in larger sizes, reflecting stainless steel's higher allowable stress and reduced need for corrosion allowance. In small sizes, 40S and SCH 40 often coincide, which makes the distinction easy to overlook. Always state the applicable standard when specifying, because ordering "SCH 40 stainless" when 40S was intended can bring a different wall thickness and cost.

Q: How do you choose the right pipe schedule?
A: Schedule selection follows from a pressure design calculation, not from habit. Start with design pressure and temperature, apply the piping code formula (ASME B31.3 for process piping, B31.1 for power, B31.4/B31.8 for pipelines) using the material's allowable stress at design temperature. Add the project corrosion allowance, typically 1.5–3 mm for carbon steel in corrosive service. Account for mill tolerance — seamless pipe carries a −12.5% wall tolerance that must be included. Consider mechanical and handling robustness for small-bore lines, erosion and velocity effects, and the minimum wall needed if the pipe will be threaded (SCH 80 is commonly the minimum for threaded carbon steel). Finally, round up to the next commercially available schedule.