; the actual size of a part will be approximately the same as the nominal size but need not be exactly the same; for example, a rod may be referred to as ¼ inch, although the actual dimension on the drawing is 0.2495 inch, and in this case ¼ inch is the nominal size....
1. In masonry, a dimension greater (by the thickness of a mortar joint) than the dimensions of the actual masonry unit; in the United States, not exceeding ½ inch (13 mm). 2. In lumber, a dimension that may vary from the actual dimensions as provided for in the local building co...
The size of pipes, fittings, flanges and valves are often given in inches as NPS - Nominal Pipe Size, or in metric units as DN - 'Diametre Nominal'.
It is essentially a “reference” size and does not translate to an exact diameter measurement of pipe 12″ and smaller. In process piping the term nominal simply refers to the name of the pipe, much like a 2″ × 4″ piece of lumber. A 2″ × 4″ board does not actually measure ...