DN (Nominal Diameter) is a standardized reference size for classifying pipes and industrial hoses, while D (Outer Diameter) is the exact external measurement in millimeters. Both matter when choosing hoses or pipes: DN ensures compatibility across fittings and systems, while D provides the precise measurement needed for proper installation and performance.
How to convert DN to inches?
To convert a DN value into inches, simply divide it by 25.4, since one inch equals 25.4 millimeters.
For example, if the pipe has a DN of 50:
50 ÷ 25.4 ≈ 1.97
This means a DN50 pipe corresponds to roughly 1.97 inches in diameter.
DN to inch conversion
DN (mm) |
Inch |
|---|---|
6 |
⅛” |
8 |
¼” |
10 |
⅜” |
15 |
½” |
20 |
¾” |
25 |
1” |
32 |
1 ¼” |
40 |
1 ½” |
50 |
2” |
65 |
2 ½” |
80 |
3” |
100 |
4” |
125 |
5” |
150 |
6” |
200 |
8” |
What does outer diameter mean?
Diameter D stands for the outer diameter of a pipe or hose, the measurement taken across its outside surface. It is usually expressed in millimeters or inches and is a key parameter when choosing parts like couplings, gaskets, or supports that must align precisely with the pipe.
International standards such as ISO, DIN, ASTM, and SMS use the outer diameter as the defining value for classification. This ensures that pipes and hoses are consistent and compatible worldwide, making integration into different systems straightforward.
Accurate knowledge of the outer diameter is vital for sealing under pressure and for selecting components that guarantee safe, reliable operation.
What does nominal diameter mean?
DN stands for nominal diameter and is a standardized term used to classify pipe sizes in plumbing, construction, and other piping-related industries.
The word “nominal” means that DN does not indicate the exact measurement of the pipe’s diameter, but rather a conventional designation that simplifies design and communication. For this reason, it is sometimes described as a “non-physical” size.
DN is always given in millimeters and represents an approximate internal diameter, without considering the pipe wall thickness.
What is the difference between D and DN diameter?
D represents the measured outer diameter of a pipe, while DN is a nominal size designation used for classification and does not always equal the actual dimensions.
For accurate design and correct component selection, it is essential that engineers and technicians understand and apply both values.
How to measure the diameter of a hose
Hoses are usually described as inner diameter × outer diameter. For example, a “4 × 8 mm silicone hose” has a 4 mm inner diameter and an 8 mm outer diameter, giving it a 2 mm wall thickness. To measure, use calipers on both the inner and outer surfaces, checking in multiple spots, as hoses may deform or wear over time.
- Choosing the right tool: The most accurate option is a caliper, though a ruler or measuring tape can be used if needed.
- Checking the outer diameter (OD): Place the caliper jaws around the outside of the hose, keeping them straight and aligned with the surface. Record the measurement.
- Checking the inner diameter (ID): If the end of the hose is open, use the smaller caliper jaws to measure the inside. Expand them gently until they touch the inner walls and note the reading.
- Taking multiple measurements: Measure in several spots along the hose, since the diameter can vary due to use, bending, or manufacturing tolerances.
- Inspecting the hose condition: Wear, deformation, or damage may affect both the accuracy of the reading and the true fit of the hose.
Practical tips
When working with pipes and hoses, engineers should always verify the outer diameter (OD) and wall thickness rather than relying only on DN, since DN is a nominal value. DN is useful when specifying or ordering components according to standards, but actual dimensions must be confirmed, especially when combining parts from different standards, to ensure proper fit and compatibility.
Understanding the distinction between DN (Nominal Diameter) and D (Outer Diameter) is key in pipe and hose selection. DN provides a standardized reference that simplifies communication and ordering, while D gives the exact measurement needed for installation and sealing. Considering both ensures correct sizing, reliable performance, and compatibility across different systems and standards.
