DN vs Pipe Outside Diameter: Why DN100 Does Not Mean a 100 mm Pipe OD
Selection explainer · English · The numerical examples below come from the AQUAFITTING 2023 English catalogue, pp. 4, 9–10. They illustrate how to read the listed rows; confirm the current document and supplied configuration before specifying a product.
A nominal-size label is a starting point for identifying a connection, not a substitute for the pipe's actual outside diameter. For a range-based mechanical coupling or flange adaptor, the listed pipe outside-diameter interval is an essential part of the selection.
A flange adaptor adds a second check: the pipe-side range and the mating flange interface must both suit the connection. Choosing a DN label alone does not resolve either check completely.
Two different ranges can share one nominal designation
The AQUAFITTING 2023 English catalogue, p. 4, lists these two Universal Flange Adaptor entries:
| Nominal designation in the source | Listed pipe outside-diameter range |
|---|---|
| DN100 | 93–117 mm |
| DN100 | 107–128 mm |
Consider a pipe whose outside diameter is 120 mm. That value is above the first entry's 117 mm upper limit, but within the second entry's 107–128 mm interval. Both entries carry the DN100 designation; only the second satisfies this particular diameter check.
This is an illustration using the source entries, not an order approval. The current product configuration, pipe material, mating flange and service conditions still need to match the application. Use the applicable current dimensional document when specifying a supplied assembly.
A flange adaptor has two interfaces
At the pipe side, identify the pipe and its applicable outside-diameter range. At the flange side, identify the mating component and its flange requirements. These are related, but they are not interchangeable dimensions.
For a coupling joining two plain-ended pipes, record both pipe ends. Do not assume that the second end has the same material, dimensions or application conditions as the first simply because the nominal sizes match.
A family maximum is not the range of every size
The same catalogue describes a MAXI family outside-diameter tolerance of up to 35 mm. Its DN100 example of 107–133 mm has an interval width of 26 mm. The individual row provides the selection range; the family headline does not enlarge that row.
The same principle applies when comparing related products. A coupling, a flange adaptor and another design generation may have similar names without sharing the same dimensional table.
Information to keep with the selection
Record the product family and exact range entry, the pipe material and outside-diameter information, the mating interface, and the technical document reference and revision. Where wall details, support components or other conditions are relevant to the selected product, retain those requirements as part of the specification.
Site information should come from reliable records or measurements obtained only where access is safe and authorised. Do not treat a photograph or a nominal-size marking alone as a complete dimensional assessment.
The practical rule
Use DN to identify the nominal connection category. Use the stated outside-diameter interval to check the pipe-side dimensional match. Then verify the remaining material, interface and configuration requirements against the current documents.
A diameter match is necessary for the relevant range selection, but it is not proof of suitability for every material or operating condition.