Free Engineering Tool
Nominal Bore Converter
Cross-reference DN (Nominal Diameter), NPS (Nominal Pipe Size), inch fraction, and actual outside diameter in mm. Select a size or search the full table.
Results
| DN | NPS | Inch Fraction | OD (mm) | OD (inch) |
|---|
What is Nominal Bore (DN)?
DN (Diamètre Nominal) is a dimensionless designator used in the metric system to describe the size of pipes, fittings, flanges, and valves. Defined by ISO 6708, it is an alphanumeric designation for reference purposes — it does not correspond exactly to any physical dimension.
What is NPS?
NPS (Nominal Pipe Size) is the North American equivalent designation, based on inches. NPS and DN describe the same pipe sizes with different numbering conventions. They share identical outside diameters per ASME B36.10.
Key Insight: The outside diameter (OD) of a pipe is fixed for a given DN/NPS. Only the wall thickness varies across different schedules (Sch 10, 40, 80, etc.), which changes the internal diameter and wall thickness.
Why DN25 ≠ 25 mm
DN values are historically based on approximate internal diameters, rounded and standardized. Over time, the actual OD was standardized to fixed values (inherited from the imperial system). DN25 corresponds to a 1-inch pipe with OD = 33.4 mm, not 25 mm.
Conversion Pattern
For sizes DN350 and above, the DN value approximately equals the OD in mm:
- DN350 → OD 355.6 mm (close to 350)
- DN400 → OD 406.4 mm (close to 400)
- DN500 → OD 508.0 mm (close to 500)
For smaller sizes, this pattern breaks down and table lookup is essential.
⚠️ Note: This table covers standard carbon steel pipe sizes per ASME B36.10 / ISO 6708. Stainless steel pipe (ASME B36.19) uses the same ODs but different wall thickness schedules (5S, 10S, 40S, 80S).
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Nikolai Shelkovenko is a vibration analysis engineer and the founder and CEO of Vibromera. For more than 15 years he has balanced rotating equipment in the field rather than on a test bench: mulchers, industrial fans, crushers, centrifuges, shafts and spindles. That work is what the Balanset instruments grew out of — they were designed as a tool a specialist can carry to the machine and use alone, on site, not as laboratory equipment. Vibromera was founded in 2017 and has been based in Porto, Portugal, since 2023. Development, assembly and support of the Balanset line all happen here. The flagship instrument is the Balanset-1A, a portable analyser for single- and two-plane balancing and for vibration diagnostics. Nikolai is personally involved in customer support, in working through difficult balancing cases and in the development of the software. He works with customers worldwide, in any language.
