Getting the pipe size wrong is one of the most expensive mistakes in industrial procurement. A steel pipe cannot be described by a single number — its specification always combines outside diameter, wall thickness, and length. Misunderstand any one of them and the pipe will not fit, will not hold the design pressure, or will cost far more than it should.
This guide walks through the complete size-specification system used in the global steel pipe trade: the three core parameters, the ASME dimension standards, the Schedule wall-thickness numbering system, NPS and DN nominal diameter conventions, a full dimension chart for quick reference, and the correct way to write a pipe specification on an inquiry or purchase order.
The Three Parameters That Define a Steel Pipe
A complete steel pipe dimension always consists of three values:
Outside Diameter (OD) — the outer width of the pipe, measured in millimeters or inches.
Wall Thickness (WT) — the thickness of the pipe wall, which directly determines pressure rating and weight.
Pipe Length — typically supplied in 6-meter (20-foot) single random lengths or 12-meter (40-foot) double random lengths.
These three numbers are not just for fitting purposes. From them, a supplier or engineer can calculate the pipe's total weight, its safe working pressure, and the cost per foot or per meter. That is why every serious inquiry should include all three — leaving any one of them open to interpretation invites pricing errors and delivery delays.
Steel Pipe Size Chart (ASME B36.10M & B36.19M)
The table below lists standard steel pipe dimensions in millimeters, covering Nominal Pipe Size (NPS), Nominal Diameter (DN), Outside Diameter (OD), and wall thickness for all common Schedule ratings and weight classes.
All wall-thickness values are in millimeters (mm). Schedules with the "S" suffix (5S, 10S, 40S, 80S) apply to stainless steel pipe per ASME B36.19M. Blank cells indicate that the schedule is not standardized for that pipe size.
| Nominal Pipe Size |
Outside Diameter (mm) |
Nominal Wall Thickness Schedule | |||||||||||||||||
| NPS | DN | OD | SCH 5s |
SCH I0s |
SCH 10 |
SCH 20 |
SCH 30 |
SCH 40s |
SCH STD |
SCH 40 |
SCH 60 |
SCH 80s |
SCH XS |
SCH 80 |
SCH 100 |
SCH 120 |
SCH 140 |
SCH 160 |
SCH XXS |
| 1/8 | 6 | 10.3 | 1.24 | 1.73 | 1.73 | 1.73 | 2.41 | 2.41 | 2.41 | ||||||||||
| 1/4 | 8 | 13.7 | 1.65 | 2.24 | |2.24 | 2.24 | 3.02 | 3.02 | 3.02 | ||||||||||
| 3/8 | 10 | 17.1 | 1.65 | 2.31 | 231 | 2.31 | 3.2 | 3.2 | 3.2 | ||||||||||
| 1/2 | 15 | 21.3 | 1.65 | 2.11 | 2.77 | 2.77 | 2.77 | 3.73 | 3.73 | 3.73 | 4.78 | 7.47 | |||||||
| 3/4 | 20 | 26.7 | 1.65 | 2.11 | 2.87 | 2.87 | 2.87 | 3.91 | 3.91 | 3.91 | 5.56 | 7.82 | |||||||
| 1 | 25 | 33.4 | 1.65 | 2.77 | 338 | 338 | 3.38 | 4.55 | 4.55 | 4.55 | 6.35 | 9.09 | |||||||
| 1 1/4 | 32 | 42.2 | 1.65 | 2.77 | 3.56 | 3.56 | 356 | 4.85 | 4.85 | 4.85 | 6.35 | 9.7 | |||||||
| 1 1/2 | 40 | 48.3 | 1.65 | 2.77 | 3.68 | 3.68 | 3.68 | 5.08 | 5.08 | 5.08 | 7.14 | 10.15 | |||||||
| 2 | 50 | 60.3 | 1.65 | 2.77 | 3.91 | 3.91 | 3.91 | 5.54 | 5.54 | 5.54 | 8.74 | 11.07 | |||||||
| 2 1/2 | 65 | 73 | 2.11 | 3.05 | 5.16 | 5.16 | 5.16 | 7.01 | 7.01 | 7.01 | 9.53 | 14.02 | |||||||
| 3 | 80 | 88.9 | 2.11 | 3.05 | 5.49 | 5.49 | 5.49 | 7.62 | 7.62 | 7.62 | 11.13 | 15.24 | |||||||
| 3 1/2 | 90 | 101.6 | 2.11 | 3.05 | 5.74 | 5.74 | 5.74 | 8.08 | 8.08 | 8.08 | |||||||||
| 4 | 100 | 114.3 | 2.11 | 3.05 | 6.02 | 6.02 | 6.02 | 8.56 | 8.56 | 8.56 | 11.13 | 13.49 | 17.12 | ||||||
| 5 | 125 | 141.3 | 2.77 | 3.4 | 6.55 | 6.55 | 6.55 | 9.53 | 953 | 9.53 | 12.7 | 15.88 | 19.05 | ||||||
| 6 | 150 | 168.3 | 2.77 | 3.4 | 7.11 | 7.11 | 7.11 | 10.97 | 10.97 | 10.97 | 14.27 | 18.26 | 21.95 | ||||||
| 8 | 200 | 219.1 | 2.77 | 3.76 | 6.35 | 7.04 | 8.18 | 8.18 | 8.18 | 10.31 | 12.7 | 12.7 | 12.7 | 15.09 | 18.26 | 20.62 | 23.01 | 22.23 | |
| 10 | 250 | 273.1 | 3.4 | 4.19 | 6.35 | 7.8 | 9.27 | 9.27 | 9.27 | 12.7 | 12.7 | 12.7 | 15.09 | 18.26 | 21.44 | 25.4 | 28.58 | 25.4 | |
| 12 | 300 | 323.9 | 3.96 | 4.57 | 6.35 | 8.38 | 9.53 | 9.53 | 10.31 | 14.27 | 12.7 | 12.7 | 17.48 | 21.44 | 25.4 | 28.58 | 33.32 | 25.4 | |
| 14 | 350 | 355.6 | 3.96 | 4.78 | 6.35 | 7.92 | 9.53 | 9.53 | 11.13 | 15.09 | 12.7 | 19.05 | 23.83 | 27.79 | 31.75 | 35.71 | |||
| 16 | 400 | 406.4 | 4.19 | 4.78 | 6.35 | 7.92 | 9.53 | 9.53 | 12.7 | i6.66 | 12.7 | 21.44 | 26.19 | 30.96 | 36.53 | 40.49 | |||
| 18 | 450 | 457.2 | 4.19 | 4.78 | 6.35 | 7.92 | 11.13 | 9.53 | 14.27 | 19.05 | 12.7 | 23.83 | 29.36 | 34.93 | 3967 | 45.24 | |||
| 20 | 5500 | 508 | 4.78 | 5.54 | 6.35 | 9.53 | 12.7 | 9.53 | 15.09 | 20.62 | 12.7 | 26.19 | 32.54 | 38.1 | 44.45 | 50.01 | |||
| 22 | 559 | 4.78 | 5.54 | 6.35 | 9.53 | 12.7 | 9.53 | 22.23 | 12.7 | 28.58 | 34.93 | 41.28 | 47.63 | 53.98 | |||||
| 24 | 6600 | 610 | 5.54 | 6.35 | 6.35 | 9.53 | 14.27 | 9.53 | 17.48 | 24.61 | 12.7 | 30.96 | 38.89 | 46.02 | 52.37 | 59.54 | |||
| 26 | 660 | 7.92 | 12.7 | 9.53 | 12.7 | ||||||||||||||
| 28 | 700 | 711 | 7.92 | 12.7 | 15.88 | 9.53 | 12.7 | ||||||||||||
| 30 | 762 | 6.35 | 7.92 | 7.92 | 12.7 | 15.88 | 9.53 | 12.7 | |||||||||||
| 32 | 800 | 813 | 7.92 | 12.7 | 15.88 | 9.53 | 17.48 | 12.7 | |||||||||||
| 34 | 884 | 7.92 | 12.7 | 15.88 | 9.53 | 17.48 | 12.7 | ||||||||||||
| 36 | 900 | 914 | 7.92 | 12.7 | 15.88 | 9.53 | 19.05 | 12.7 | |||||||||||
| 38 | 965 | 9.53 | 12.7 | ||||||||||||||||
| 40 | 1000 | 1016 | 9.53 | 12.7 | |||||||||||||||
| 42 | 1067 | 12.7 | 15.88 | 9.53 | 19.05 | 12.7 | |||||||||||||
| 44 | 1100 | 1118 | 9.53 | 12.7 | |||||||||||||||
| 46 | 1168 | 9.53 | 12.7 | ||||||||||||||||
| 48 | 1200 | 1219 | 9.53 | 12.7 | |||||||||||||||
| 52 | 1321 | ||||||||||||||||||
| 56 | 1422 | ||||||||||||||||||
| 60 | 1524 | ||||||||||||||||||
Dimension Standards: ASME B36.10M vs B36.19M
Two ASME standards govern the dimensional specifications of welded and seamless steel pipe: ASME B36.10M for carbon and alloy steel pipe, and ASME B36.19M for stainless steel pipe. Both define outside diameter and wall thickness, but they differ in which schedule ratings are standardized.
ASME B36.10M — Carbon and Alloy Steel Pipe
ASME B36.10M covers the standardization of dimensions for welded and seamless carbon and alloy steel pipe used in high- or low-temperature and pressure service. It is important to distinguish "pipe" from "tube" under this standard: pipe is intended for pipeline systems transporting fluids such as oil, gas, water, and slurry, whereas tube follows a different dimensional convention.
A key characteristic of B36.10M: for pipe sizes smaller than NPS 12 (DN 300), the actual outside diameter is larger than the NPS nominal value. For NPS 14 and above, the OD in inches numerically equals the NPS. For steel tube, by contrast, the actual outside diameter equals the nominal size at every dimension.
Why Pipe Sizes Below NPS 12 Have a Mismatched OD
This discrepancy is a historical artifact. Early pipe sizing was based on a standard outside diameter, but because every pipe has a wall, the inside diameter always differs from the outside. In small pipe sizes, the wall thickness is significant relative to the diameter, so OD and ID diverge noticeably. As pipe size increases, wall thickness becomes proportionally smaller, and OD and ID converge.
Because no direct mathematical relationship exists between the legacy outside-diameter convention and the Nominal Pipe Size system, ASME B36.10M accepts both designation methods.
Wall Thickness Designations in B36.10M
ASME B36.10M identifies wall thickness in two ways. The older system uses weight class designations: Standard (STD), Extra-Strong (XS), and Double Extra-Strong (XXS). The modern system uses Schedule numbers: 5, 10, 20, 30, 40, 60, 80, 100, 120, 140, and 160.
ASME B36.19M — Stainless Steel Pipe
ASME B36.19M applies to stainless steel pipe dimensions, both seamless and welded. Its framework is largely identical to B36.10M, with one critical difference: stainless-steel-specific schedule ratings carry the suffix "S" because their wall thickness values differ from the carbon-steel equivalents.
The stainless-only schedules are:
NPS 14 through NPS 22 (DN 350–550): Schedule 10S
NPS 12: Schedule 40S
NPS 10 and NPS 12: Schedule 80S
Always include the "S" suffix when specifying stainless steel pipe schedule. Sch 40 (carbon) and Sch 40S (stainless) are not interchangeable — their wall thickness values differ at certain sizes.
Three Common Ways to Specify a Steel Pipe
In international trade, pipe dimensions are expressed using three complementary systems. Depending on the region and the buyer's convention, any combination may appear on an inquiry:
Wall thickness via Pipe Schedule — e.g., "schedule 40 steel pipe," "schedule 80 pipe." This is the most common method globally.
Diameter via NPS or DN — Nominal Pipe Size (inches, North American system) or Nominal Diameter (millimeters, metric system).
Weight via WGT class — expressed in LB/FT (pounds per foot) or KG/M (kilograms per meter), common in North and South America.
What Is Pipe Schedule and How Is It Calculated?
Pipe schedule (abbreviated Sch.) is a dimensionless numbering system for wall thickness, originally defined in ASME B36.10 and subsequently adopted by many other pipe standards.
The ASME definition captures the engineering logic: because steel pipe's primary application is transporting fluids under pressure, the internal diameter — called the nominal bore (NB) — is the critical flow dimension. When a pipe carries pressurized fluid, its wall must have sufficient strength and thickness. Wall thickness is therefore specified in Schedules, abbreviated SCH, as the given standard and definition for pipe schedule.
The schedule number is derived from the design pressure and the material's allowable stress:
Sch = P / [δ]t × 1000
Where:
P = Design pressure (MPa)
[δ]t = Allowable stress of the material at design temperature (MPa)
In practice, schedule numbers are not calculated for every order — they are selected from the standardized series. The formula explains why a higher schedule number corresponds to a thicker wall: a higher design pressure relative to allowable stress produces a larger schedule value.
What Does SCH Actually Mean for Wall Thickness?
A frequent source of confusion: SCH is not a wall thickness value — it is a wall thickness series. For any given NPS, each schedule number maps to a specific, standardized wall thickness in millimeters or inches. Change the schedule, and the wall thickness changes, even though the outside diameter stays the same.
The most commonly encountered schedule ratings are:
Sch 5, 5S, 10, 10S, 20, 20S, 30, 40, 40S, 60, 80, 80S, 100, 120, 140, 160.
The relationship is straightforward: the larger the schedule number, the thicker the pipe wall, and the higher the pressure the pipe can withstand. For reference, at NPS 4 (DN 100, OD 114.3 mm), Sch 40 has a 6.02 mm wall while Sch 80 has 8.56 mm — a 42% increase in wall thickness for the same outer diameter.
Why Schedule 40 and Schedule 80 Dominate the Market
Newcomers to the pipe industry often ask why sch 40 and sch 80 appear in nearly every project and catalog. The answer is practical rather than technical: these two schedules cover the pressure ranges encountered in the vast majority of industrial applications.
Schedule 40 handles medium-pressure scenarios — water supply, drainage, standard process piping, compressed air, and low-pressure steam. It strikes a balance between pressure capacity and material cost.
Schedule 80 is specified for higher-pressure service — oil and gas gathering, chemical processing, high-pressure steam, and hydraulic systems.
Because demand is concentrated in these two ratings, they are the most widely stocked, have the shortest lead times, and typically offer the best per-ton pricing. Buyers specifying non-standard schedules (Sch 30, Sch 60, Sch 100, etc.) should expect longer delivery and a price premium.
Importantly, schedule rating is independent of material grade. A sch 40 pipe can be manufactured from stainless steel (e.g., ASTM A312 Grade 316L) or carbon steel (e.g., API 5L, ASTM A53, ASTM A106B, A179, A252, A333, and others). The schedule defines the geometry, not the metallurgy.
Nominal Pipe Size (NPS) Explained
Nominal Pipe Size (NPS) is the North American standard set of pipe sizes, used for both high- and low-pressure and temperature service. It is a dimensionless designation based on inches — NPS 4 means a 4-inch nominal pipe, not a pipe with a 4-inch outside diameter.
Under the NPS system, a pipe is always specified with two dimensionless numbers: the NPS (size, based on inches) and the Schedule (wall thickness rating, abbreviated Sched. or Sch.). For example, "NPS 4 Sch 40" fully defines the pipe's outer diameter and wall thickness without needing to state either dimension explicitly.
Nominal Diameter (DN) Explained
Nominal Diameter (DN) is the metric-system equivalent, used to designate the general size of pipes and pipeline fittings. Because pipe walls are relatively thin, the inside and outside diameters are close in value; the average of the two is adopted as the nominal diameter.
The practical purpose of DN is interchangeability. Pipes and fittings (flanges, elbows, tees, valves) that share the same DN are designed to connect. This is why a DN100 flange fits a DN100 pipe regardless of manufacturer — the DN system ensures dimensional compatibility across the supply chain.
DN is a nominal designation, not the actual inside diameter. A DN100 pipe does not have a 100 mm internal bore — its actual ID depends on the wall thickness (schedule). DN50, for example, represents a pipe with a nominal diameter of 50 mm and an outside diameter of 60.3 mm.
DN is written as the letters "DN" followed by a numerical value, with the unit understood as millimeters. Common examples: DN15, DN25, DN50, DN100, DN200.
NPS to DN Conversion Table
Converting between NPS (inch-based) and DN (mm-based) is not a simple unit conversion for small pipe sizes — the relationship is non-linear below DN100 due to the historical OD convention discussed earlier. There are three rules to remember.
1. Small sizes (DN 6 through DN 80) — memorize or look up
| DN (mm) | 6 | 8 | 10 | 15 | 20 | 25 | 32 | 40 | 50 | 65 | 80 |
| NPS (inch) | 1/8 | 1/4 | 3/8 | 1/2 | 3/4 | 1 | 1 1/4 | 1 1/2 | 2 | 2 1/2 | 3 |
2. DN100 and larger — use the approximation formula
DN ≈ 25 × NPS DN-25×NPS
For example, NPS 4 ≈ DN100, NPS 6 ≈ DN150, NPS 8 ≈ DN200, NPS 12 ≈ DN300. This approximation is widely accepted in commercial specifications.
3. Exact inch-to-mm conversion
For dimensional calculations (not nominal size conversion), the exact conversion is:
1 inch = 25.4 mm
Use this when converting actual measured dimensions such as outside diameter or wall thickness from inches to millimeters.
Pipe Weight Class: STD, XS, XXS
The Weight Class (WGT) system is an older method of indicating wall thickness that remains in common use today, particularly in legacy specifications and certain regional markets. It consists of three grades:
STD (Standard) — the baseline wall thickness for each pipe size.
XS (Extra-Strong) — a thicker wall for higher-pressure service.
XXS (Double Extra-Strong) — the thickest standard wall, for the highest pressure applications.
The evolution of these grades reflects the history of the pipe industry. In the earliest days of pipe production, each size had only one specification — the standard tube (STD). As higher-pressure applications emerged, thicker pipes designated XS were introduced. XXS followed to handle even more demanding high-pressure fluids.
Later, as materials and processing technology advanced, the market demanded more cost-effective thin-wall options. This eventually led to the introduction of the Schedule numbering system, which offers far more granular wall-thickness choices than the three-grade WGT system. The correspondence between pipe schedules and weight classes for each size is defined in ASME B36.10 and ASME B36.19.
For many common sizes, STD aligns with Sch 40 and XS aligns with Sch 80 — but this is not universal. Always verify the actual wall thickness against the dimension chart, especially for sizes NPS 12 and above where the equivalency breaks down.
How to Write a Steel Pipe Specification Correctly
A well-written pipe specification leaves no room for supplier interpretation. Four notation formats are accepted in international trade, and buyers may encounter any of them depending on regional convention.
a. Outside Diameter × Wall Thickness
The most explicit format, stating actual measured dimensions. Example:
Φ 88.9 mm × 5.49 mm (3 1/2" × 0.216"), length 6 m (20 ft) or 12 m (40 ft). May also specify Single Random Length (SRL, typically 18–25 ft) or Double Random Length (DRL, typically 38–40 ft).
b. NPS × Schedule
The most common global format. Example: NPS 3 inch × Sch 40, or NPS 4 inch × Sch 40. This defines the same pipe as the example in (a) above.
c. NPS × Weight Class
Used in legacy and certain regional specifications. Example: NPS 3 inch × SCH STD, or NPS 4 inch × SCH STD.
d. Outside Diameter × Weight per Foot (lb/ft)
Commonly used in North America and South America. Example: a pipe with an outside diameter of 3 1/2" would be described as having a weight of 16.8 lb/ft. The unit "lb/ft" stands for pounds per foot, and the supplier derives the wall thickness from the published weight-per-foot tables.
For procurement clarity, Royal Steel Group recommends format (b) — NPS × Schedule — supplemented with material grade (e.g., ASTM A106 Gr. B) and standard length. This is the least ambiguous and most widely understood format in global steel pipe trade.
How to Write a Steel Pipe Specification Correctly
A well-written pipe specification leaves no room for supplier interpretation. Four notation formats are accepted in international trade, and buyers may encounter any of them depending on regional convention.
a. Outside Diameter × Wall Thickness
The most explicit format, stating actual measured dimensions. Example:
Φ 88.9 mm × 5.49 mm (3 1/2" × 0.216"), length 6 m (20 ft) or 12 m (40 ft). May also specify Single Random Length (SRL, typically 18–25 ft) or Double Random Length (DRL, typically 38–40 ft).
b. NPS × Schedule
The most common global format. Example: NPS 3 inch × Sch 40, or NPS 4 inch × Sch 40. This defines the same pipe as the example in (a) above.
c. NPS × Weight Class
Used in legacy and certain regional specifications. Example: NPS 3 inch × SCH STD, or NPS 4 inch × SCH STD.
d. Outside Diameter × Weight per Foot (lb/ft)
Commonly used in North America and South America. Example: a pipe with an outside diameter of 3 1/2" would be described as having a weight of 16.8 lb/ft. The unit "lb/ft" stands for pounds per foot, and the supplier derives the wall thickness from the published weight-per-foot tables.
For procurement clarity, Royal Steel Group recommends format (b) — NPS × Schedule — supplemented with material grade (e.g., ASTM A106 Gr. B) and standard length. This is the least ambiguous and most widely understood format in global steel pipe trade.
Royal Steel Group supplies seamless steel pipe and welded steel pipe in all standard schedules and sizes, from NPS 1/8 to NPS 48, certified to API 5L, ASTM A53, A106, A312, and other international standards. Send us your specification for a competitive quotation.
Frequently Asked Questions
What is the difference between schedule 40 and schedule 80 pipe?
Schedule 80 pipe has a thicker wall than schedule 40 for the same NPS, allowing it to withstand higher pressure. For example, at NPS 4 (DN 100, OD 114.3 mm), sch 40 has a wall thickness of 6.02 mm while sch 80 has 8.56 mm. The outside diameter remains identical; only the wall thickness and internal bore change.
Is NPS the same as outside diameter (OD)?
Only for NPS 14 and above. For NPS 1/8 through NPS 12, the actual outside diameter is larger than the NPS number. For example, NPS 4 has an OD of 114.3 mm (4.5 inches), not 101.6 mm (4 inches). This is a historical convention defined in ASME B36.10M.
What does DN mean in pipe size?
DN stands for Nominal Diameter, a metric designation in millimeters for pipe and fitting sizes. DN ensures interchangeability — pipes and fittings with the same DN can be connected. DN is a nominal value, not the actual inside diameter. DN50 corresponds approximately to NPS 2 and has an outside diameter of 60.3 mm.
How do I convert NPS to DN?
For sizes DN100 and above, use the approximation DN ≈ 25 × NPS (e.g., NPS 4 = DN100, NPS 6 = DN150). For smaller sizes (DN 6–80), the conversion is non-linear and must be looked up in a conversion table — e.g., NPS 1/2 = DN15, NPS 1 = DN25, NPS 2 = DN50, NPS 3 = DN80.
What is the standard length of steel pipe?
The most common standard lengths are 6 meters (20 feet, single random length / SRL) and 12 meters (40 feet, double random length / DRL). SRL typically ranges from 18 to 25 feet, and DRL from 38 to 40 feet. Fixed lengths (e.g., exactly 6 m or 12 m) are also available but may carry a cutting premium.
What does the 'S' mean in schedule 40S or 80S?
The "S" suffix indicates a stainless steel pipe schedule per ASME B36.19M. Stainless schedules (5S, 10S, 40S, 80S) have different wall thickness values than their carbon-steel equivalents (without the S) at certain pipe sizes. Always include the S when specifying stainless steel pipe to avoid receiving the wrong wall thickness.
Are STD and XS the same as Sch 40 and Sch 80?
For many common sizes below NPS 12, STD is equivalent to Sch 40 and XS is equivalent to Sch 80. However, this equivalency does not hold for all sizes — particularly NPS 12 and above, where STD and Sch 40 diverge. Always verify the actual wall thickness against the ASME B36.10 dimension chart rather than assuming equivalence.
Post time: Aug-24-2026