API 5L specifies requirements for steel line pipe, while ASME B36.10M provides a standardized dimensional system for welded and seamless wrought steel pipe. In practice, API 5L pipe dimensions may be specified using NPS, outside diameter (OD), wall thickness (WT), and Schedule, depending on the project and procurement requirements.
API 5L has its own dimensional requirements, including specified outside diameter and wall thickness. ASME B36.10M should therefore be treated as a dimensional reference rather than a replacement for the API 5L product specification.
The ASME B36.10M Dimension System for API 5L Pipe
Understanding the relationship between NPS, DN, OD, WT, and Schedule is essential when specifying API 5L line pipe.
| Dimension | Meaning | Function |
| NPS | Nominal Pipe Size | Identifies the nominal pipe size |
| DN | Nominal Diameter | Metric designation corresponding to nominal size |
| OD | Outside Diameter | Standardized outside diameter associated with the pipe size |
| WT | Wall Thickness | Nominal thickness of the pipe wall |
| Schedule | Wall-thickness designation | Identifies standardized wall-thickness options for applicable NPS sizes |
For example, NPS 6 corresponds to DN 150 and has a standardized outside diameter of 168.3 mm (6.625 in.) under the ASME B36.10M dimensional system.
The basic relationship can be summarized as:
NPS → OD → Schedule / WT
For API 5L procurement, however, the pipe should not be identified by NPS or Schedule alone. The required API 5L grade, PSL, OD, wall thickness, pipe type, length, and applicable project requirements should be clearly specified.
API 5L Outside Diameter Dimensions According to ASME B36.10M
Outside diameter is one of the key dimensions used to identify API 5L line pipe. NPS is a nominal designation, while each NPS corresponds to a standardized OD.
Typical dimensions include:
| NPS | DN | OD (mm) | OD (in.) |
| 2 | DN 50 | 60.3 | 2.375 |
| 3 | DN 80 | 88.9 | 3.500 |
| 4 | DN 100 | 114.3 | 4.500 |
| 6 | DN 150 | 168.3 | 6.625 |
| 8 | DN 200 | 219.1 | 8.625 |
| 10 | DN 250 | 273.0 | 10.750 |
| 12 | DN 300 | 323.9 | 12.750 |
| 16 | DN 400 | 406.4 | 16.000 |
| 20 | DN 500 | 508.0 | 20.000 |
| 24 | DN 600 | 610.0 | 24.000 |
For example, an NPS 10 API 5L pipe has a standardized OD of 273.0 mm, not an outside diameter of 10 inches.
The actual measured outside diameter is subject to the dimensional tolerances specified by the applicable product standard and project specification. Therefore, the nominal NPS and standardized OD should be distinguished from the allowable manufacturing tolerance.
API 5L Wall Thickness and Schedule According to ASME B36.10M
For a given NPS, the standardized OD generally remains fixed while the nominal wall thickness changes with the selected Schedule.
For example, NPS 10 has an OD of 273.0 mm, while different Schedule designations correspond to different nominal wall thicknesses:
| NPS | DN | OD (mm) | Schedule | Nominal WT (mm) |
| 10 | 250 | 273.0 | SCH 40 | 9.27 |
| 10 | 250 | 273.0 | SCH 80 | 15.09 |
| 10 | 250 | 273.0 | SCH 100 | 18.26 |
| 10 | 250 | 273.0 | SCH 120 | 22.23 |
| 10 | 250 | 273.0 | SCH 140 | 25.40 |
| 10 | 250 | 273.0 | SCH 160 | 28.58 |
This shows that Schedule is a wall-thickness designation, not an API 5L steel grade or PSL designation.
For example:
- API 5L X60 identifies the line pipe specification and steel grade.
- NPS 10 identifies the nominal pipe size.
- 273.0 mm OD identifies the standardized outside diameter.
- SCH 80 / 15.09 mm WT identifies a particular dimensional option.
These designations describe different aspects of the pipe and should not be treated as interchangeable.
It is also important to note that Schedule is not a universal thickness option for every NPS. The available combinations of NPS, OD, Schedule, and nominal wall thickness depend on the applicable dimensional standard.
API 5L ASME B36.10M Dimensions Chart
The following chart combines common pipe dimensions used when discussing API 5L line pipe with the ASME B36.10M dimensional system.
| NPS | DN | OD (mm) | OD (in.) | Schedule | Nominal WT (mm) |
| 2 | 50 | 60.3 | 2.375 | SCH 40 | 3.91 |
| 3 | 80 | 88.9 | 3.500 | SCH 40 | 5.49 |
| 4 | 100 | 114.3 | 4.500 | SCH 40 | 6.02 |
| 6 | 150 | 168.3 | 6.625 | SCH 40 | 7.11 |
| 8 | 200 | 219.1 | 8.625 | SCH 40 | 8.18 |
| 10 | 250 | 273.0 | 10.750 | SCH 40 | 9.27 |
| 12 | 300 | 323.9 | 12.750 | SCH 40 | 10.31 |
| 16 | 400 | 406.4 | 16.000 | SCH 40 | 12.70 |
| 20 | 500 | 508.0 | 20.000 | SCH 40 | 12.70 |
| 24 | 600 | 610.0 | 24.000 | SCH 40 | 12.70 |
This chart shows commonly referenced dimensions for API 5L line pipe using the ASME B36.10M dimensional system, including NPS, DN, outside diameter, Schedule and nominal wall thickness. The listed values are representative dimensions and should not be treated as a complete API 5L dimensional table.
For procurement, NPS or OD alone does not fully define an API 5L pipe. Buyers should also specify the required grade, PSL, pipe type, wall thickness, length and other project-specific requirements.
The final dimensional requirements should be confirmed against the applicable API 5L edition, ASME B36.10M requirements where referenced, project specification and purchase order.
API 5L ASME B36.10M Dimensions FAQs
What is the relationship between API 5L and ASME B36.10M?
API 5L is a product specification for steel line pipe, covering requirements such as grades, PSL, manufacturing, inspection and testing. ASME B36.10M is a dimensional standard for welded and seamless wrought steel pipe. They serve different purposes and may be used together when specifying pipe.
Does API 5L use ASME B36.10M dimensions?
API 5L has its own dimensional requirements, including specified outside diameter and wall thickness. ASME B36.10M provides a standardized dimensional framework that can be used or referenced for pipe size and wall-thickness designations where applicable. The applicable API 5L and project requirements should always be checked.
Does Schedule determine API 5L wall thickness?
Schedule identifies a standardized nominal wall-thickness option for a particular NPS. However, API 5L procurement requirements should identify the required outside diameter and wall thickness, together with the applicable product and project specifications. Schedule should not be treated as a substitute for a complete API 5L dimensional requirement.
