Steel pipes are specified in a variety of ways around the world, depending on the standards used. Because these standards (such as ANSI/ASME, ISO, DIN, and GB/T) employ different systems for nominal size, wall thickness, and material designation, engineers, procurement specialists, and fabricators must be familiar with the correct expression methods. A well-formed dimension specification ensures compatibility between design drawings, purchase orders, and on-site installation, and prevents costly misunderstandings.
This guide explains the key elements of steel pipe dimension notation, compares major standards, and clarifies the relationships between nominal size, wall thickness, and weight grades.
Common Expression Formats
In practice, steel pipe dimensions are typically expressed as a combination of Nominal Diameter (DN or NPS) + Wall Thickness Grade (SCH) + Material Standard.
The table below shows the most widely used formats:
| Expression Method | Standard Source | Example |
| DN + SCH + Material (Metric/International) | ISO, GB | DN100 Sch.40 ASTM A106 Gr.B |
| NPS + SCH + Material (Imperial) | ASME/ANSI | 4″ Sch.80 ASTM A312 TP316 |
| OD × Wall Thickness + Material (Direct dimensions) | ISO, GB | 114.3 × 6.02 ASTM A106 Gr.B |
Understanding these formats is easier if we first break down what each element—NPS/DN, SCH, OD, wall thickness, and material—actually means in the context of piping standards.
Size Designation Systems
Nominal Pipe Size (NPS) and Nominal Diameter (DN)
- NPS (inch-based, ANSI/ASME): A nominal size identifier; actual OD differs from the stated inches (e.g., NPS 6 → OD 168.3 mm).
- DN (mm-based, ISO/metric): Approximate metric equivalent (DN150 ≈ NPS 6).
- Note: NPS and DN are reference identifiers, not physical measurements. The actual OD and wall thickness must be verified in the relevant standard tables.
Small Diameters (Require Direct Memorization)
| 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 |
Approximate Conversion Formula for DN ≥ 100
- DN ≈ 25 × NPS
- NPS ≈ DN ÷ 25
- This formula is applicable only when DN ≥ 100 and serves as a quick engineering approximation.
Precise Conversion
- 1 inch = 25.4 millimeters
- When precision is required, always use 25.4 instead of 25 to minimize cumulative error.
Schedule Number (SCH)
- Indicates wall thickness grade.
- Higher SCH number → thicker wall → higher pressure capacity.
- The same SCH represents different wall thickness values depending on NPS.
With NPS/DN defining the outer diameter and SCH defining the wall thickness, the next logical step is to understand the role of OD, wall thickness, and material standards in the full specification.
OD, Wall Thickness, and Materials
Outside Diameter (OD)
- Actual physical outer diameter of the pipe.
- Fixed for a given NPS, regardless of wall thickness.
Wall Thickness (WT)
- Can be specified by SCH or in millimeters.
- Formula for internal diameter:
- 𝐼𝐷=𝑂𝐷−2×𝑊𝑇
Material Standards
- ASTM A106 Gr.B → Seamless carbon steel (U.S.)
- ASTM A312 TP304 → Stainless steel (U.S.)
- GB/T 8163 20# → Seamless carbon steel (China)
The American standards ANSI/ASME B36.10M and B36.19M are the most widely referenced for OD and wall thickness. Understanding their differences is critical when converting between carbon steel and stainless steel specifications.
ANSI/ASME B36.10M vs. B36.19M
| Aspect | B36.10M | B36.19M |
| Materials | Carbon steel, alloy steel | Stainless steel |
| Applications | General piping | Corrosive or sanitary piping |
| Wall Thickness | Sch. series (5–XXS) | Sch. series + S series (5S, 10S, 40S, 80S) |
| “S” Thickness | N/A | Slightly thinner than equivalent Sch. |
Example: NPS 4
B36.10M Sch.40 → 6.02 mm
B36.19M 40S → 5.74 mm
Before Schedule numbers became the dominant method, the Weight Grade (WGT) system—STD, XS, and XXS—was widely used. These terms still appear on older drawings and equipment specifications.
Pipe Weight Grade (WGT) System
| Grade | Full Name | Features | Applications |
| STD | Standard | Thinner wall | General service |
| XS | Extra Strong | Thicker than STD | Medium/high pressure |
| XXS | Double Extra Strong | Thickest | Extreme pressure |
Example for NPS 4:
- STD → 4.27 mm (~Sch.40)
- XS → 5.16 mm (~Sch.80)
- XXS → 7.11 mm (~Sch.160)
Summary
A complete steel pipe dimension should be written as:
[Nominal Size (NPS or DN)] + [Wall Thickness (SCH or mm)] + [Material Standard]
Example: NPS 4″ Sch.80 ASTM A106 Gr.B or DN100 × 6 mm ASTM A312 TP316.
EN 
DE 
RU 
FA 
ID 
ES 
FR 
NL