What is DN designation?

• A. Diameter nominal size
• B. Metric nominal size
• C. Dynamic nominal size
• D. Diagonal nominal size

Answer: (B)

DN designation identifies the nominal diameter used in metric piping standards. It stands for Diameter Nominal and is expressed in millimeters, giving a standardized reference for the bore size category of pipes, fittings, and valves. The DN value is a nominal, approximate size and does not specify the exact inner bore or outer diameter, which depend on the wall thickness and schedule. This is why it’s called the metric nominal size: it uses metric units to classify pipe components in a consistent, global system, unlike inches-based sizing such as NPS. For example, a DN 50 designation corresponds to a nominal bore around 50 mm, not a precise measurement of the actual bore or outer diameter.