Use this steel pipe size chart to check common nominal pipe sizes, outside diameters and metric equivalents for steel pipe purchasing. It also explains how pipe schedule, wall thickness and inside diameter relate to the specified size.
The tables provide a quick reference for common carbon and alloy steel pipe dimensions. Confirm the applicable standard, material grade and required wall thickness before placing an order.
Nominal Pipe Size Chart
The nominal pipe size chart lists common NPS designations, DN equivalents and actual outside diameters in inches and millimeters. NPS is a nominal designation rather than a direct measurement of the pipe’s outside diameter.
| NPS (in.) | DN | Outside Diameter (in.) | Outside Diameter (mm) |
|---|---|---|---|
| 1/8 | 6 | 0.405 | 10.3 |
| 1/4 | 8 | 0.540 | 13.7 |
| 3/8 | 10 | 0.675 | 17.1 |
| 1/2 | 15 | 0.840 | 21.3 |
| 3/4 | 20 | 1.050 | 26.7 |
| 1 | 25 | 1.315 | 33.4 |
| 1 1/4 | 32 | 1.660 | 42.2 |
| 1 1/2 | 40 | 1.900 | 48.3 |
| 2 | 50 | 2.375 | 60.3 |
| 2 1/2 | 65 | 2.875 | 73.0 |
| 3 | 80 | 3.500 | 88.9 |
| 3 1/2 | 90 | 4.000 | 101.6 |
| 4 | 100 | 4.500 | 114.3 |
| 5 | 125 | 5.563 | 141.3 |
| 6 | 150 | 6.625 | 168.3 |
| 8 | 200 | 8.625 | 219.1 |
| 10 | 250 | 10.750 | 273.0 |
| 12 | 300 | 12.750 | 323.9 |
| 14 | 350 | 14.000 | 355.6 |
| 16 | 400 | 16.000 | 406.4 |
| 18 | 450 | 18.000 | 457.2 |
| 20 | 500 | 20.000 | 508.0 |
| 24 | 600 | 24.000 | 609.6 |
DN values are nominal metric designations. Dimensions shown are representative of standard steel pipe sizes; verify against the applicable specification.
Pipe Schedule and Wall Thickness
A pipe schedule chart is used to identify the nominal wall thickness associated with a given pipe size. The required thickness depends on both NPS and schedule, so specifying NPS alone is not enough to define a complete pipe dimension.
When checking steel pipe wall thickness, confirm:
- NPS: The required nominal pipe size.
- Schedule: The specified wall thickness series.
- Wall thickness: The nominal thickness required by the applicable dimensional standard.
For complete size-by-size data, refer to our Schedule 40 Steel Pipe Dimensions and Schedule 80 Steel Pipe Dimensions guides.
For applicable carbon and alloy steel pipe, dimensional references such as ASME B36.10M can be used to verify outside diameter and nominal wall thickness. Always follow the standard specified for the project.
Steel Pipe Inside Diameter
The steel pipe inside diameter (ID) depends on the outside diameter and wall thickness. For a nominal calculation, use:
Inside Diameter = Outside Diameter − (2 × Wall Thickness)
For example, an NPS 2 pipe with an outside diameter of 2.375 inches and a nominal wall thickness of 0.154 inches has a calculated inside diameter of approximately 2.067 inches.
| Dimension | Value |
|---|---|
| NPS | 2 |
| Outside diameter | 2.375 in. |
| Nominal wall thickness | 0.154 in. |
| Calculated inside diameter | 2.067 in. |
The calculated ID is a reference value, not a guaranteed measured dimension. Manufacturing tolerances, internal coatings and linings may affect the usable bore.
Steel Pipe Size Conversion: Inches and Millimeters
Steel pipe specifications may use both imperial and metric units. Use the following distinctions when checking a steel pipe size conversion chart:
- NPS: Nominal pipe size, commonly designated in inches.
- DN: Nominal metric size designation.
- OD: Actual outside diameter, expressed in inches or millimeters.
- Wall thickness: The specified nominal thickness, expressed in inches or millimeters.
To convert a measured dimension from inches to millimeters, multiply by 25.4. For example, an outside diameter of 2.375 inches is approximately 60.3 mm.
This conversion applies to measured dimensions, not directly to NPS or DN designations. When comparing supplier quotations or project drawings, confirm the nominal size, actual OD and wall thickness together.
Steel Pipe Specifications for Purchasing
A complete steel pipe specification helps suppliers confirm the required product and prepare an accurate quotation.
| Specification | Details to Confirm |
|---|---|
| Pipe size | NPS, DN or specified outside diameter |
| Wall thickness | Schedule number or required thickness |
| Material standard and grade | For example, ASTM A53 or ASTM A106, as specified |
| Pipe type | Seamless or welded, where required |
| Length | Common supply length or specified cut length |
| Quantity | Required quantity and unit of measure |
| Additional requirements | Surface treatment, inspection documents, marking and export packing, where applicable |
Common supply lengths may include 6 m and 12 m, depending on pipe type, size, applicable standard and stock availability. Confirm the required length before ordering.
Material standards should match the intended service and project requirements. For example, ASTM A53 Steel Pipe and ASTM A106 Steel Pipe have different scopes and requirements; they should not be treated as interchangeable simply because their dimensions may overlap.
For a steel pipe quotation, provide the required size, schedule or wall thickness, material grade, length and quantity. These details help establish whether the proposed supply matches your specification.
Frequently Asked Questions
What information is needed to identify a steel pipe size?
Start with the NPS or specified OD, then confirm the required schedule or wall thickness. The material standard and grade should also be included when requesting a quotation.
Can pipes with the same NPS have different inside diameters?
Yes. Pipes with the same NPS can have different wall thicknesses. The specified thickness determines the calculated inside diameter, subject to applicable dimensional tolerances.
Which standard should I use for steel pipe dimensions?
Use the dimensional standard required by the project or product specification. ASME B36.10M covers dimensions for applicable welded and seamless wrought steel pipe; other product types and materials may use different standards.