🔗 PDF datasheet measurement units and dimensional standards
Millimetres, inches and nominal dimensions: the basics
In day-to-day work with hydraulic systems and marine components, a significant part of the misunderstandings does not stem from technical errors but from confusion between distinct concepts: actual units of measurement, nominal designations, and the reference systems used by different international standards.
To work correctly with pipes, valves, fittings and flanges, it is necessary to clearly distinguish between:
- units of measurement (millimetres, inches)
- nominal dimensions (DN, PN)(DN, PN)
- classification systems (metric, imperial)
- applicable international technical standards (ISO, EN, ASME, etc.)
Standards organisations
After World War II, international technical bodies were established to develop shared standards across countries. Over time these organisations evolved into standards authorities responsible for defining rules covering design, manufacturing, testing, safety, quality and environmental management.
Today technical standardisation governs product and process requirements in every industrial sector worldwide. The main reference organisations are listed below.
📌 ISO • International Organization for Standardization
Founded in 1947 and headquartered in Geneva, ISO is the leading non-governmental organisation for the development of international technical standards. ISO standards apply worldwide.
Each country may adopt ISO standards as national standards. The designation varies depending on the adoption path:
- UNI ISO • when adopted exclusively at Italian national level
- UNI EN ISO • when adopted at both European and national level
ISO standards use the metric system. Dimensions are expressed in millimetres, centimetres or metres.
📌 EN • European Standards
The abbreviation EN identifies standards developed by CEN (European Committee for Standardization). All European Union member states are required to adopt them. In Italy they become UNI EN standards.
National standards cannot conflict with EN standards: the purpose of European harmonisation is to establish common technical requirements throughout Europe, eliminating conflicts between national standards.
EN standards use the metric system. Dimensions are expressed in millimetres.
📌 ASME • American Society of Mechanical Engineers
ASME is a non-profit professional organisation founded in 1880 in the United States. It develops standards and technical guidelines in the field of mechanical engineering and industrial systems.
ASME standards use the imperial system. Dimensions are expressed in inches or feet.
📌 Other national standards organisations
ANSI • American National Standards Institute Private non-profit organisation responsible for industrial standards in the United States.
AFNOR • Association Française de Normalisation French national standards organisation.
DIN • Deutsches Institut für Normung German standards organisation.
UNI • Ente Italiano di Normazione Italian standards organisation responsible for developing and publishing technical standards across industrial, commercial and service sectors. UNI represents Italy within ISO and CEN.
BS • British Standards Standards published by the BSI Group, officially recognised as the United Kingdom National Standards Body.
The metric system and the imperial system
Technical standards use one of two measurement systems, depending on the country of origin, the industrial sector and the applicable standards.
In piping systems and mechanical installations, both systems remain widely used for historical and technical reasons.
📏 The inch
The inch is the unit of length in the imperial system, used mainly in the United States and other English-speaking countries.
1 inch = 25.4 mm (exact correspondence)
📏 The millimetre
The millimetre is the unit of length in the metric system. It corresponds to one thousandth of a metre.
📏 Nominal dimensions
The most frequent confusion in marine components does not concern units of measurement as such, but nominal designations. DN and PN are the two most common cases.
These values are standardised reference indicators and do not necessarily correspond to exact physical measurements.
🔘 DN • Nominal Diameter
DN is a conventional designation that identifies the nominal internal passage size of a component.
DN provides a standardised reference for matching pipes, fittings, valves and flanges.
Example: DN 50 indicates a nominal diameter corresponding approximately to 50 mm internal passage. The actual internal diameter may vary depending on wall thickness, manufacturing standard and pressure class.
DN values can be correlated to both metric dimensions and imperial pipe sizes through dedicated conversion tables.
DN is therefore not an exact measurement, but a standardised dimensional designation.
🔘 PN • Nominal Pressure
PN is a conventional designation indicating the allowable operating pressure of a component under specified reference conditions.
PN values are expressed in bar.
Example: PN 16 indicates a nominal operating pressure of 16 bar under standard reference conditions.
PN is not a physical dimension. It is a classification parameter linked to:
- mechanical strength
- wall thickness
- flange dimensions
- number and size of bolt holes
PN ratings are conventionally defined for water service at temperatures not exceeding 20 °C and not lower than 0 °C.
🔘 Difference between DN and PN
DN and PN describe completely different characteristics:
| DN | PN | |
| Describes | Nominal internal passage size | Allowable operating pressure |
| Used for | Dimensional compatibility | Mechanical resistance |
Two components intended to be directly connected must have compatible DN values and compatible PN ratings, to ensure dimensional compatibility, sealing integrity and mechanical safety.
🔘 How PN affects component geometry
The pressure rating has a direct effect on component geometry, because higher operating pressures require:
- increased wall thickness
- reinforced structures
- different flange drilling configurations
- materials with higher mechanical resistance
PN therefore conditions the mechanical design of pipes, valves, flanges and fittings, even though it is not itself a dimensional measurement.