This conversion chart helps you easily compare Nominal Pipe Size (NPS) values with their fractional, decimal, metric (mm), and DN (diamètre nominal) equivalents. Use it as a quick reference to find accurate pipe size correlations across international standards.
Nominal Pipe Size (NPS) is a standardized term used to describe the size of pipes in the imperial (inch-based) system. It’s important to understand that nominal size refers to a named designation, not the actual physical dimension of the pipe.
For example:
An NPS 2-inch pipe doesn’t measure exactly 2.000 inches in diameter. Instead, it has a fixed outside diameter (OD) of approximately 2.375 inches (60.3 mm), while the inside diameter (ID) varies depending on the pipe’s schedule (wall thickness).
This sizing convention originates from historical water pipe systems, where the nominal number represented the approximate internal diameter before modern standardization.
The conversion chart bridges the gap between imperial and metric pipe sizing systems. It lists the nominal size alongside its fractional, decimal (inch), millimeter, and DN equivalents for easy cross-reference.
|
Column |
Description |
|
Nominal (NPS) |
The standard U.S. designation for pipe size. It is a label, not a literal measurement. |
|
Fraction (inches) |
The traditional fractional representation of the nominal size (e.g., 1 ½″, 2 ¼″). |
|
Decimal (inches) |
The same size expressed as a decimal value for engineering or CAD use. |
|
Metric (mm) |
The approximate size in millimeters, based on conversion (1 in = 25.4 mm). |
|
DN (Diamètre Nominal) |
The international metric equivalent, used in ISO and EN standards. DN values are rounded and do not always match exact millimeter dimensions. |
To find the correct size:
Locate the Nominal (NPS) value in the first column.
Read across to find its equivalent in fractional, decimal, metric, and DN units.
Keep in mind that DN values and metric equivalents are nominal. They’re designed for reference, not precision measurement.
If exact dimensions are needed for fabrication or fitting, refer to ASME or ISO pipe dimension tables for the specific schedule.
Example:
NPS 2 corresponds to an OD of 2.375 inches (60.3 mm) and is typically identified as DN 50 in metric systems.
This means that a “2-inch” pipe and a “DN 50” pipe are equivalent designations, though not exact dimensional matches.
