Nps Nominal Pipe Sizes
Reference data and engineering information about nps nominal pipe sizes for fluid mechanics applications.
Overview
Nominal Pipe Size (NPS) is the North American designation for pipe sizing. In the metric system, the equivalent is DN (Diamètre Nominal), defined by ISO 6708 as an alphanumeric reference to bore or outside diameter expressed in millimeters. The nominal number is not equal to the actual measured dimension—especially above 12 inches.
Common pipe materials include galvanized steel, black steel, copper, cast iron, concrete, ABS, PVC, CPVC, polyethylene, and polybutylene.
Sizing Terminology
- NPS – Nominal Pipe Size; references the inside diameter of the pipe.
- IPS – Iron Pipe Size; an older designation also referencing approximate inside diameter.
- DIPS – Ductile Iron Pipe Size; references inside diameter.
- CTS – Copper Tube Size; references outside diameter of the tube.
Unit Converter
The original page includes a unit converter alongside the NPS/DN reference data. Use the converter below for quick inch-to-millimeter checks when comparing nominal pipe sizes and outside diameters.
Pipe diameter unit converter
Outside Diameter Comparison
Outside diameters vary by standard. The table below shows ODs for common nominal sizes across four widely used specifications.
DN(mm) | Inch | ISO 6708(mm) | DIN EN 10220(mm) | DIN EN 10255(mm) | ASME(mm) |
|---|---|---|---|---|---|
| 15 | 1/2 | 21.3 | 20 | 21.3 | 21.3 |
| 20 | 3/4 | 26.9 | 25 | 26.9 | 26.7 |
| 25 | 1 | 33.7 | 30 | 33.7 | 33.4 |
| 32 | 1-1/4 | 42.4 | 38 | 42.4 | 42.2 |
| 40 | 1-1/2 | 48.3 | 44.5 | 48.3 | 48.3 |
| 50 | 2 | 60.3 | 57 | 60.3 | 60.3 |
| 80 | 3 | 88.9 | 88.9 | 88.9 | 88.9 |
| 100 | 4 | 114.3 | 108 | 114.3 | 114.3 |
| 150 | 6 | 168.3 | 159 | 168.3 | 168.3 |
| 200 | 8 | 219.1 | 216 | 219.1 | 219.1 |
Source: engineeringtoolbox.com
Standard ANSI/ASME B36.10 Pipe Dimensions
The table below lists outside diameters and inside diameters for Schedule 40 and Schedule 80 carbon steel pipes per ASME B36.10.
NPS | OD(mm) | Sch 40 ID(mm) | Sch 80 ID(mm) |
|---|---|---|---|
| 1/8 | 10.3 | 6.8 | 5.5 |
| 1/4 | 13.7 | 9.2 | 7.7 |
| 3/8 | 17.1 | 12.5 | 10.7 |
| 1/2 | 21.3 | 15.8 | 13.8 |
| 3/4 | 26.7 | 20.9 | 18.5 |
| 1 | 33.4 | 26.6 | 24.3 |
| 1-1/4 | 42.2 | 35.1 | 32.5 |
| 1-1/2 | 48.3 | 40.9 | 38.1 |
| 2 | 60.3 | 52.5 | 49.3 |
| 2-1/2 | 73 | 62.7 | 59 |
| 3 | 88.9 | 77.9 | 73.7 |
| 4 | 114.3 | 102.3 | 97.2 |
| 6 | 168.3 | 154.1 | 146.4 |
| 8 | 219.1 | 202.7 | 193.7 |
| 10 | 273.1 | 254.5 | 244.5 |
Source: engineeringtoolbox.com
Restored Original Source Tables
The following tables are restored from the original source page to preserve the complete reference data.
ISO 6708 - Pipework components - DN (Nominal Pipe Size NPS)
The original Engineering ToolBox table contains 28 engineering data rows from DN 10 through DN 1200. Shared site layout/search rows from the extraction are intentionally excluded because they are not pipe-size data.
DN(mm) | Inch | ISO 6708Pipework components | DIN EN 10220Seamless Steel Pipes | DIN EN 10255 Threaded Tube | ASME |
|---|---|---|---|---|---|
| 10 | 3/8 | 17.2 | |||
| 15 | 1/2 | 21.3 | 20 | 21.3 | 21.3 |
| 20 | 3/4 | 26.9 | 25 | 26.9 | 26.7 |
| 25 | 1 | 33.7 | 30 | 33.7 | 33.4 |
| 32 | 1 1/4 | 42.4 | 38 | 42.4 | 42.2 |
| 40 | 1 1/2 | 48.3 | 44.5 | 48.3 | 48.3 |
| 50 | 2 | 60.3 | 57 | 60.3 | 60.3 |
| - | 2 1/2 | - | - | 73 | 73 |
| 65 | - | 76.1 | 76.1 | 76.1 | - |
| 80 | 3 | 88.9 | 88.9 | 88.9 | 88.9 |
| - | 3 1/2 | - | - | 101.6 | 101.6 |
| 100 | 4 | 114.3 | 108 | 114.3 | 114.3 |
| 125 | - | 139.7 | 133 | 139.7 | - |
| - | 5 | - | - | 141.3 | 141.3 |
| 150 | 6 | 168.3 | 159 | 168.3 | 168.3 |
| 200 | 8 | 219.1 | 216 | 219.1 | 219.1 |
| 250 | 10 | 273 | 267 | 273 | 273 |
| 300 | 12 | 323.9 | 318 | 323.9 | 323.8 |
| 350 | 14 | 355.6 | 368 | 355.6 | 355.6 |
| 400 | 16 | 406.4 | 419 | 406.4 | 406.4 |
| 450 | 18 | 457 | 470 | 457 | 457 |
| 500 | 20 | 508 | 521 | 508 | 508 |
| 600 | 24 | 610 | 622 | 610 | 610 |
| 700 | 28 | 711 | 720 | 711 | 711 |
| 800 | 32 | 813 | 820 | 813 | 813 |
| 900 | 36 | 914 | 920 | 914 | 914 |
| 1000 | 40 | 1016 | 1020 | 1016 | 1016 |
| 1200 | 48 | 1220 | 1219 | 1219 |
Source: engineeringtoolbox.com
DN / NPS Outside Diameter Comparison
Engineering Notes
- NPS ≠ actual diameter. For sizes 1/8 through 12, NPS roughly equals the bore. For sizes 14 inches and above, NPS equals the outside diameter in inches.
- DN is not a direct conversion. DN is a dimensionless reference number; do not multiply it by 25.4 to get millimeters.
- Wall thickness schedules. Schedule 40 (standard weight) and Schedule 80 (extra heavy) are the most common. Higher schedule numbers mean thicker walls and smaller inside diameters for the same OD.
- CTS exception. Copper tube sizes are based on outside diameter, so inside diameter depends on the tube type (K, L, M).
- Standards vary. DIN EN 10220 seamless steel pipe ODs are often smaller than ISO 6708 or ASME values at the same nominal size. Always verify which standard applies to your fittings and flanges.
- Thread compatibility. ASME B1.20.1 (NPT) threads are specified on the nominal pipe size, not the measured diameter.