Nominal pipe size is a size designation used to identify a pipe within a particular dimensional system. It is not a promise that a ruler will show that number as the pipe's inside or outside diameter. The pipe family, material standard and wall designation supply the dimensions needed to identify compatible parts.
Nominal pipe size is a size designation used to identify a pipe within a particular dimensional system. It is not a promise that a ruler will show that number as the pipe's inside or outside diameter. The pipe family, material standard and wall designation supply the dimensions needed to identify compatible parts.
A name and a measurement do different jobs
Manufacturers publish tables connecting nominal sizes to actual dimensions. Charlotte Pipe's dimensional catalogs list outside diameter and wall dimensions for their specific pipe products. The Copper Development Association's handbook separately lists nominal or standard tube sizes and actual outside diameters.
For a separate fitting-geometry measurement, see the fitting laying length entry.
Those tables are useful because pipe and tubing use different naming conventions. Even the same fractional size on two packages can refer to products with different outside dimensions. Identify the applicable dimensional family before comparing the numbers.
The inside opening can also differ for products with the same outside diameter. A thicker wall leaves less bore within that outside dimension. Some outside-diameter-controlled plastic pipe identifies that relationship through standard dimension ratio, which must be read with the product standard. That matters to flow and to the connection's actual design.
Size does not establish service approval
A fitting can physically match a pipe's dimensions while lacking the required rating or approval for its use. Pressure piping, drainage piping and appliance tubing can have different material, connection and service requirements.
A nominal-size label also does not prove that a system was sized correctly. Demand, pipe length, available pressure, grade where applicable and the approved design determine the required line size. Our water supply sizing reference addresses that separate calculation.
Do not order a replacement solely from the measured bore of an old fitting. Deposits, manufacturing tolerances and a damaged end can distort that measurement. Existing markings and the manufacturer's dimensional table provide better identification evidence. The pipe ovality entry separates cross-section roundness from the nominal-size designation.
Reading the complete specification
Ask for the nominal size together with the material, dimensional family, wall designation and connection type. For a transition between materials, the quote should identify both actual outside dimensions and the approved adapter. If a reducer bushing is proposed, identify both connection sizes and its permitted application as well.
The CTS and IPS sizing entry explains two common size families. Pipe schedule explains a wall-series designation. Nominal size helps name a component; those additional details establish which component it actually is.