When working on construction, plumbing, mechanical, or industrial projects, selecting the right size of steel pipe is critical. Using an incorrect size can lead to weak connections, fluid leakage, or unnecessary costs. That’s where a steel pipe size chart becomes an essential reference tool.
This guide covers everything you need to know about steel pipe sizing — from how pipes are measured to understanding the size chart, conversions, and practical uses.
Why Steel Pipe Size Matters
Steel pipes are used in multiple industries, including:
Plumbing systems for water and gas supply.
Construction projects like scaffolding and structural supports.
Industrial applications such as oil, chemical, and energy pipelines.
Automotive and mechanical engineering for hydraulic and fuel systems.
If the pipe size doesn’t match the design requirements, the entire system may fail. For example, a smaller diameter pipe may restrict fluid flow, while a larger pipe could increase costs and reduce efficiency.
How Are Steel Pipes Measured?
Unlike other materials, steel pipe measurement can be confusing. Two main terms are used:
Nominal Pipe Size (NPS) – A standardized system used in North America that refers to the approximate inside diameter of the pipe.
Outside Diameter (OD) – The actual measured distance across the pipe’s outer wall.
Wall Thickness / Schedule (SCH) – Indicates the thickness of the pipe’s wall. Common schedules include SCH 40, SCH 80, and SCH 160. A higher schedule number means thicker walls.
Inside Diameter (ID) – The internal measurement of the pipe, which is affected by wall thickness.
👉 Example:
A pipe labeled as NPS 2, SCH 40 does not mean the pipe’s inside diameter is exactly 2 inches. The actual OD is 2.375 inches, and the ID varies based on wall thickness.
Steel Pipe Size Chart (Nominal vs Actual)
Below is a simplified steel pipe size chart for common sizes (in inches).
| Nominal Pipe Size (NPS) | Outside Diameter (OD) | Inside Diameter (ID) (SCH 40) | Inside Diameter (ID) (SCH 80) |
|---|---|---|---|
| 1/2 inch | 0.84 in | 0.622 in | 0.546 in |
| 3/4 inch | 1.05 in | 0.824 in | 0.742 in |
| 1 inch | 1.315 in | 1.049 in | 0.957 in |
| 1 1/2 inch | 1.90 in | 1.610 in | 1.500 in |
| 2 inch | 2.375 in | 2.067 in | 1.939 in |
| 3 inch | 3.50 in | 3.068 in | 2.900 in |
| 4 inch | 4.50 in | 4.026 in | 3.826 in |
| 6 inch | 6.625 in | 6.065 in | 5.761 in |
| 8 inch | 8.625 in | 7.981 in | 7.625 in |
| 10 inch | 10.75 in | 10.020 in | 9.562 in |
| 12 inch | 12.75 in | 11.938 in | 11.374 in |
This chart shows how nominal size differs from actual OD and ID.
Understanding Pipe Schedules
The term “schedule” defines the thickness of a steel pipe’s wall. Thicker walls mean the pipe can handle higher pressures.
Schedule 40 (SCH 40): Standard thickness, commonly used for plumbing and general applications.
Schedule 80 (SCH 80): Thicker wall, suitable for higher-pressure systems.
Schedule 160 & XXS (Double Extra Strong): Very thick walls for extremely high-pressure environments.
👉 Example:
A 2-inch SCH 40 pipe has a wall thickness of 0.154 in, while a 2-inch SCH 80 pipe has 0.218 in thickness.
Conversion Between NPS and DN
In international projects, you’ll often see DN (Diameter Nominal) instead of NPS.
DN is based on the metric system (millimeters).
Conversion: NPS 1 inch ≈ DN 25, NPS 2 inch ≈ DN 50, and so on.
This makes it easier for engineers and manufacturers in different regions to align pipe standards.
Steel Pipe Sizes in Millimeters (DN to OD)
| DN (mm) | NPS (inches) | Outside Diameter (mm) |
|---|---|---|
| DN 15 | 1/2 in | 21.3 mm |
| DN 20 | 3/4 in | 26.7 mm |
| DN 25 | 1 in | 33.4 mm |
| DN 40 | 1 1/2 in | 48.3 mm |
| DN 50 | 2 in | 60.3 mm |
| DN 80 | 3 in | 88.9 mm |
| DN 100 | 4 in | 114.3 mm |
| DN 150 | 6 in | 168.3 mm |
| DN 200 | 8 in | 219.1 mm |
| DN 250 | 10 in | 273.0 mm |
| DN 300 | 12 in | 323.8 mm |
Common Applications of Steel Pipe Sizes
Different pipe sizes are chosen depending on application:
Residential Plumbing – Small sizes (1/2″, 3/4″) for water lines.
Gas Lines – Usually 1/2″ to 2″ depending on pressure and demand.
Industrial Piping – Large diameters (6″, 8″, 12″) for oil, gas, and chemical flow.
Structural Support – Hollow pipes with larger OD are used in construction scaffolding and framework.
Mechanical & Automotive – Medium sizes (1″ to 4″) are common in hydraulic and exhaust systems.
Practical Example of Pipe Sizing
Imagine you’re designing a water pipeline that needs to deliver 500 liters per minute.
A 1-inch SCH 40 pipe may not handle the required flow efficiently.
A 2-inch SCH 40 pipe has a larger inside diameter, reducing friction and supporting the flow better.
If the system involves high pressure, you may need a 2-inch SCH 80 instead.
This is why understanding the chart is important — it saves cost, avoids leaks, and ensures efficiency.
Tips for Using a Steel Pipe Size Chart
Always check both OD and ID – Nominal size alone can be misleading.
Consider pressure rating – Higher pressures need thicker walls (higher schedule).
Account for corrosion allowance – In chemical industries, slightly larger pipes may be chosen to allow for internal corrosion.
Use conversion charts when needed – Especially in international projects involving DN and NPS.
Steel Pipe Standards Around the World
Steel pipe sizing is not the same everywhere. Different countries follow different standards, which is why size charts are essential for international projects.
ANSI/ASME (American National Standards Institute / American Society of Mechanical Engineers): Widely used in the USA, defining NPS and schedules.
ISO (International Organization for Standardization): Provides global guidelines, especially using DN (Diameter Nominal).
DIN (Deutsches Institut für Normung): German standards, often referenced in Europe.
BS (British Standards): Used in the UK and Commonwealth countries.
JIS (Japanese Industrial Standards): Applied in Asia and industrial manufacturing.
If you’re sourcing steel pipes from different regions, a conversion chart is crucial to avoid mismatched fittings.
Differences Between Steel Pipes and Tubes
A common confusion arises between steel pipes and steel tubes. While both are hollow and round, there are key differences:
Pipes are measured by nominal size (NPS/DN) and are mainly designed for transporting fluids or gases.
Tubes are measured by outside diameter (OD) and wall thickness, often used in structural and mechanical applications.
👉 Example: A 2-inch steel pipe and a 2-inch steel tube do not have the same dimensions. That’s why checking a size chart is essential.
Modern Uses of Steel Pipe Sizing
Today, steel pipes aren’t just used in plumbing or oil pipelines. With innovations in engineering, they’re found in:
Renewable energy projects – transporting steam and fluids in solar and geothermal plants.
Automotive industry – exhaust systems, chassis components, and hydraulic lines.
Agriculture – irrigation and water distribution.
DIY and fabrication – furniture frames, gym equipment, and home projects.
In all these areas, selecting the right pipe size ensures safety, durability, and cost-effectiveness.
Conclusion
A steel pipe size chart is not just a table of numbers — it’s a critical reference that ensures accuracy, efficiency, and safety in any project involving steel pipes. Whether you’re working on plumbing, construction, oil and gas, or mechanical applications, knowing how to read and apply these charts helps you choose the right pipe size every time.
From nominal pipe size (NPS) and diameter nominal (DN) to wall thickness schedules, these details may seem technical, but they are the backbone of reliable engineering and construction.
So next time you’re about to select a steel pipe, pull out the size chart — it might save you from costly mistakes and ensure your system works exactly as intended.
If you want to know how to calculate sizes, read our detailed guide: How to Calculate Any Size: A Complete Guide.