Nominal Diameter

Published: May 7, 2017 | Last updated: July 5, 2023

What Does Nominal Diameter Mean?

Nominal Diameter is also known as the mean or average outside diameter and is represented by DN. It is neither equal to the inner diameter (ID) nor the outer diameter (OD) of the pipe. Nominal is a word that denotes non-specificity and in this case, identifies the approximate inner diameter with a non-dimensional number.

Pipes having identical nominal diameters can be easily connected or interchanged with each other. Pipes are available in different DN sizes and the DN is used to arrive at the pipe dimensions using standard tables and schedules.

The actual inner diameter of pipes from different manufacturers can deviate by several millimeters and such pipes can be combined if the DN is indicated with reference to standards.

Trenchlesspedia Explains Nominal Diameter

The value of the nominal diameter is close to the inner diameter of the pipe but not equal to it, and is adopted in order to unify the connecting dimensions of the pipe and pipe fittings. The nominal diameter is specified using DN followed by a dimensionless number which corresponds to the approximate inner diameter of the pipe.

Generally, a pipe has an outer, inner, and nominal diameter, where the nominal diameter is the one used for design drawings.This standard is convenient for designing and manufacturing and also for specifying the name of the pipe or pipe fitting.

Pipes are manufactured from a variety of materials such as concrete, cast iron, polyethylene (PVC), high-density polyethylene (HDPE), and glass-reinforced plastic (GRP). For plumbing works, pipe size is referred to as nominal pipe size (NPS) whose metric equivalent is the nominal diameter (DN) and conform to the International Standards Organization (ISO).

The terms nominal bore (NB) and DN are used interchangeably with NPS. DN is the European equivalent of NPS but shows pipe sizes differently than NPS. A 2-inch pipe may simply be referred to as DN 50 and to convert NPS to DN it has to be multiplied by 25.

Based on the NPS and pipe schedule, the OD and pipe thickness can be obtained from standard reference tables. For a specified NPS, the OD remains constant and the wall thickness increases with the schedule. Similarly, for a given schedule, the OD will increase with NPS while thickness stays constant or increases.

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