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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.
What Is Nominal Pipe Size (NPS)?
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.
How the Conversion Chart Works
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.
Understanding Each Column: Nominal, Fraction, Decimal, Metric, DN
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Column
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Description
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Nominal (NPS)
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The standard U.S. designation for pipe size. It is a label, not a literal measurement.
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Fraction (inches)
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The traditional fractional representation of the nominal size (e.g., 1 ½″, 2 ¼″).
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Decimal (inches)
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The same size expressed as a decimal value for engineering or CAD use.
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Metric (mm)
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The approximate size in millimeters, based on conversion (1 in = 25.4 mm).
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DN (Diamètre Nominal)
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The international metric equivalent, used in ISO and EN standards. DN values are rounded and do not always match exact millimeter dimensions.
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How to Read the Table: Tips & Examples
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.
Full Conversion Table: Nominal Pipe to Fraction, Decimal, Metric, DN
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Why Nominal Pipe Size Doesn’t Always Match the Physical Dimensions
It’s a common misconception that a pipe’s nominal size equals its actual diameter. In reality:
- For NPS sizes 12 inches and smaller, the outside diameter (OD) is fixed for each nominal size, regardless of schedule (wall thickness).
- The inside diameter (ID) decreases as the pipe wall gets thicker — meaning higher schedules (e.g., Sch 80, Sch 160) have smaller internal diameters than lighter ones (e.g., Sch 40).
- For sizes above NPS 14, the nominal value and actual OD begin to align more closely.
This system ensures that pipes of the same nominal size will fit the same fittings and flanges, even if they have different wall thicknesses.
Why It’s Important to Convert NPS to Fraction, Decimal, Metric & DN
Understanding how NPS relates to metric and DN values is crucial for engineers, contractors, and suppliers who work across different standards.
Key reasons for accurate conversion:
- Global compatibility: International projects often require matching imperial pipe sizes (NPS) with metric specifications (DN).
- Procurement clarity: Many suppliers list dimensions in either NPS or DN. Having both references prevents ordering errors.
- Design and calculation accuracy: Hydraulic, flow, and pressure calculations often use metric or decimal units, making conversions essential.
- Quality control: On-site verification often uses millimeter measurements — having metric equivalents helps confirm correct installation.
Common Mistakes & FAQs in Pipe Size Conversion
“Nominal 2 inch” vs. Actual OD/ID
An NPS 2 pipe does not have an OD of 2 inches. Its actual OD is 2.375 inches (60.3 mm). Always check the OD and wall thickness for precise fitting and performance calculations.
Metric vs DN vs NPS Confusion
- Metric (mm): Actual dimensional measurement.
- DN (Diamètre Nominal): Rounded metric designation — a convenient label, not a measured value.
- NPS (Nominal Pipe Size): The imperial labeling system used primarily in North America.
- Use DN and NPS interchangeably as reference labels, not as exact measurement indicators.
What Schedules Do & Why They Don’t Change OD
The term “schedule” refers to pipe wall thickness.
- Increasing the schedule (e.g., from Sch 40 to Sch 80) increases wall thickness and reduces internal diameter.
- However, the outside diameter remains the same for a given NPS.
This standardization ensures compatibility across different pipe grades and pressures.
What to Keep in Mind When Converting Pipe Sizes
Understanding pipe size conversions is essential for accurate design, installation, and procurement. Nominal Pipe Size (NPS) is a standardized naming system used in the imperial measurement system, but it does not directly represent the pipe’s actual physical dimensions.
- For NPS sizes 12 inches and smaller, the outside diameter remains fixed while the inside diameter varies according to the pipe’s wall thickness or schedule.
- For mixed systems, always verify both the outside and inside diameters from standard tables before selecting fittings or flanges. Relying solely on nominal labels can lead to mismatched components and costly installation issues.
Use this conversion chart as a quick reference for identifying equivalent pipe sizes across unit systems, but consult detailed dimensional standards (ASME, ISO, or EN) when precise measurements are required.
