JRVAL
Jul 31 2025
Table of Contents
The Ultimate Pipe Size Conversion Guide
Navigating global engineering projects requires fluency in both imperial and metric systems. This definitive guide demystifies pipe sizing, providing a clear and comprehensive conversion chart from NPS and fractional inches to DN and millimeters, covering sizes up to DN 3600.
Decoding the Standards
NPS vs. DN: The Nominal Identifiers
Nominal Pipe Size (NPS) is the North American set of standard sizes for pipes used for high or low pressures and temperatures. The size is specified with a dimensionless number in inches (e.g., NPS 2). DN (Diamètre Nominal) is the international and European equivalent, using millimeters as its reference (e.g., DN 50). It is crucial to understand that both NPS and DN are reference numbers, not precise measurements of the pipe's diameter. Their primary purpose is to standardize connections, ensuring that pipes, fittings, and valves from different manufacturers are interoperable.
Fraction, Decimal, and Millimeter: The Real Numbers
While NPS and DN are for identification, the actual physical dimensions are expressed in more precise terms. Fractional Inches (e.g., 2 ½") are the common way to refer to imperial sizes in the field. Decimal Inches (e.g., 2.500") are the engineering equivalent, essential for calculations and technical drawings. Millimeters (mm) are the direct metric measurement, providing the highest level of precision and forming the basis for the DN system. For engineering and fabrication, the pipe's actual Outer Diameter (OD) is the most critical dimension.
Complete Pipe Size Conversion Chart
Use this comprehensive table to quickly convert between all major pipe sizing standards. The conversion is based on the standard formula: 1 inch = 25.4 mm.
| NPS (Inch) | Decimal (in) | Metric (mm) | DN |
|---|---|---|---|
| 1/8" | 0.125 | 3.175 | 6 |
| 1/4" | 0.250 | 6.35 | 8 |
| 3/8" | 0.375 | 9.525 | 10 |
| 1/2" | 0.500 | 12.7 | 15 |
| 3/4" | 0.750 | 19.05 | 20 |
| 1" | 1.000 | 25.4 | 25 |
| 1 ¼" | 1.250 | 31.75 | 32 |
| 1 ½" | 1.500 | 38.1 | 40 |
| 2" | 2.000 | 50.8 | 50 |
| 2 ½" | 2.500 | 63.5 | 65 |
| 3" | 3.000 | 76.2 | 80 |
| 4" | 4.000 | 101.6 | 100 |
| 5" | 5.000 | 127.0 | 125 |
| 6" | 6.000 | 152.4 | 150 |
| 8" | 8.000 | 203.2 | 200 |
| 10" | 10.000 | 254.0 | 250 |
| 12" | 12.000 | 304.8 | 300 |
| 14" | 14.000 | 355.6 | 350 |
| 16" | 16.000 | 406.4 | 400 |
| 18" | 18.000 | 457.2 | 450 |
| 20" | 20.000 | 508.0 | 500 |
| 24" | 24.000 | 609.6 | 600 |
| 28" | 28.000 | 711.2 | 700 |
| 30" | 30.000 | 762.0 | 750 |
| 32" | 32.000 | 812.8 | 800 |
| 36" | 36.000 | 914.4 | 900 |
| 42" | 42.000 | 1066.8 | 1050 |
| 48" | 48.000 | 1219.2 | 1200 |
| 54" | 54.000 | 1371.6 | 1350 |
| 60" | 60.000 | 1524.0 | 1500 |
| 72" | 72.000 | 1828.8 | 1800 |
| 84" | 84.000 | 2133.6 | 2100 |
| 96" | 96.000 | 2438.4 | 2400 |
| 108" | 108.000 | 2743.2 | 2700 |
| 120" | 120.000 | 3048.0 | 3000 |
| 144" | 144.000 | 3657.6 | 3600 |
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Note: The relationship between NPS and DN is established by standards like ASME B36.10M and ISO 6708. While the direct conversion is 1 inch = 25.4 mm, the DN designation is a rounded nominal value used for reference. For critical engineering applications, always refer to the specific pipe schedule and its corresponding actual outer diameter (OD). |
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