Steel pipe sizing can be confusing - with multiple naming systems (DN, NPS, NB), schedule numbers (SCH), and measurement units (metric and imperial), it's easy to make mistakes that lead to ordering the wrong pipe. This comprehensive guide explains all the sizing systems, provides complete dimension charts, and helps you confidently specify the right steel pipe for any application.
Understanding Steel Pipe Sizing Systems
NPS - Nominal Pipe Size (Imperial)
NPS is the North American standard for pipe sizing. The "nominal" size is neither the inside diameter (ID) nor the outside diameter (OD) - it's a dimensionless designator. For NPS 1/8 through NPS 12, the NPS number is roughly the inside diameter in inches. For NPS 14 and larger, the NPS number equals the outside diameter in inches.
Important: The outside diameter is fixed for each NPS size, regardless of wall thickness. Only the inside diameter changes with different schedules.
DN - Diamètre Nominal (Metric)
DN is the international (ISO) metric equivalent of NPS. DN values are approximately the NPS value multiplied by 25. For example, NPS 2 = DN50, NPS 4 = DN100, NPS 6 = DN150.
Schedule (SCH) - Wall Thickness Designation
The Schedule number indicates the wall thickness of the pipe. Higher schedule numbers mean thicker walls and higher pressure ratings. Common schedules include:
SCH 5 / SCH 5S: Thinnest wall (stainless steel)
SCH 10 / SCH 10S: Light wall
SCH 40 / SCH 40S: Standard weight (most common)
SCH 80 / SCH 80S: Extra heavy (high pressure)
SCH 160: Very heavy (extreme pressure)
XXS (Double Extra Strong): Thickest available
Complete Steel Pipe Size Chart
Below is the complete reference chart for the most commonly used steel pipe sizes. All dimensions are based on ASME B36.10M (welded and seamless steel pipe) and ASME B36.19M (stainless steel pipe).
| NPS | DN | OD (mm) | OD (inch) | SCH | Wall (mm) | Weight (kg/m) |
|---|---|---|---|---|---|---|
| 1/2" | 15 | 21.3 | 0.840 | SCH 40 | 2.77 | 1.27 |
| 1/2" | 15 | 21.3 | 0.840 | SCH 80 | 3.73 | 1.62 |
| 3/4" | 20 | 26.7 | 1.050 | SCH 40 | 2.87 | 1.69 |
| 1" | 25 | 33.4 | 1.315 | SCH 40 | 3.38 | 2.50 |
| 1-1/4" | 32 | 42.2 | 1.660 | SCH 40 | 3.56 | 3.39 |
| 1-1/2" | 40 | 48.3 | 1.900 | SCH 40 | 3.68 | 4.05 |
| 2" | 50 | 60.3 | 2.375 | SCH 40 | 3.91 | 5.44 |
| 2-1/2" | 65 | 73.0 | 2.875 | SCH 40 | 5.16 | 8.63 |
| 3" | 80 | 88.9 | 3.500 | SCH 40 | 5.49 | 11.29 |
| 4" | 100 | 114.3 | 4.500 | SCH 40 | 6.02 | 16.07 |
| 6" | 150 | 168.3 | 6.625 | SCH 40 | 7.11 | 28.26 |
| 8" | 200 | 219.1 | 8.625 | SCH 40 | 8.18 | 42.55 |
| 10" | 250 | 273.0 | 10.750 | SCH 40 | 9.27 | 60.32 |
| 12" | 300 | 323.8 | 12.750 | SCH 40 | 10.31 | 79.73 |
How to Calculate Steel Pipe Weight
For procurement and logistics planning, it's essential to calculate the weight of steel pipe. The formula for calculating the weight per meter of a round steel pipe is:
Weight (kg/m) = (OD - WT) × WT × 0.0246615
Where:
OD = Outer Diameter (mm)
WT = Wall Thickness (mm)
0.0246615 = constant for steel density (7.85 g/cm³)
Example: For a 2" SCH 40 pipe (OD = 60.3mm, WT = 3.91mm):
Weight = (60.3 - 3.91) × 3.91 × 0.0246615 = 56.39 × 3.91 × 0.0246615 = 5.44 kg/m
To calculate total weight: Total Weight = Weight per meter × Pipe Length × Quantity
Standard Pipe Lengths
| Type | Standard Length | Notes |
|---|---|---|
| Single Random | 4.8 - 6.7m (16-22 ft) | Common for carbon steel |
| Double Random | 9.1 - 12.2m (30-40 ft) | Preferred for long runs |
| Cut Lengths | Custom (1-12m) | Available on request |
| Standard Fixed | 6m or 12m | Most common for export |
Choosing the Right Schedule
| Application | Recommended Schedule | Pressure Rating |
|---|---|---|
| Low-pressure water/drainage | SCH 40 / SCH 40S | Up to 150 PSI |
| General structural | SCH 40 | - |
| Medium-pressure fluid | SCH 80 / SCH 80S | Up to 300 PSI |
| High-pressure steam/gas | SCH 160 | Up to 600 PSI |
| Extreme pressure | XXS | Up to 1000+ PSI |
| Stainless steel (light) | SCH 5S / SCH 10S | Low pressure, sanitary |
NPS to DN Conversion Quick Reference
| NPS (inch) | DN (mm) | OD (mm) |
|---|---|---|
| 1/4" | 8 | 13.7 |
| 3/8" | 10 | 17.1 |
| 1/2" | 15 | 21.3 |
| 3/4" | 20 | 26.7 |
| 1" | 25 | 33.4 |
| 1-1/2" | 40 | 48.3 |
| 2" | 50 | 60.3 |
| 3" | 80 | 88.9 |
| 4" | 100 | 114.3 |
| 6" | 150 | 168.3 |
| 8" | 200 | 219.1 |
| 12" | 300 | 323.8 |
Frequently Asked Questions
What is the difference between NPS and NB?
NPS (Nominal Pipe Size) and NB (Nominal Bore) are essentially the same - both refer to the nominal size designation of a pipe. NB is more commonly used in British and Commonwealth standards, while NPS is the American standard. A 2" NPS pipe is the same as a 2" NB pipe - both have an OD of 60.3mm.
What does SCH 40 mean?
SCH 40 (Schedule 40) is a wall thickness designation in the ASME B36.10 standard. It is the most common "standard weight" pipe schedule. For example, a 2" SCH 40 pipe has an OD of 60.3mm and a wall thickness of 3.91mm.
How do I convert DN to NPS?
Divide the DN value by 25 to get the approximate NPS in inches. For example, DN100 ÷ 25 = 4, so DN100 = NPS 4". More precisely, DN = NPS × 25.4, but the rounded value (×25) is commonly used in practice.
Conclusion
Understanding steel pipe sizing systems - NPS, DN, and Schedule - is essential for correctly specifying and ordering steel pipe. By using the charts and formulas in this guide, you can confidently select the right pipe dimensions for any application, whether you need galvanized steel pipe, carbon steel pipe, or stainless steel pipe.
At Tianjin Kun Yu Technology, we manufacture steel pipe in all standard sizes from 1/2" to 12" (DN15-DN300) with various schedules. Custom sizes and specifications are available on request. Contact us for a free quotation.
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