The designation a product is sold by, which is not necessarily any dimension it has.
Nominal size is a naming convention, and the gap between it and reality varies by product family. A 2×4 is 1.5 × 3.5 in; a 1/2 in pipe has a 0.84 in outside diameter; a 1/8 BSP thread is 0.383 in across. None of these are errors — the nominal figure identifies the product line, and the actual dimensions are tabulated separately.
The rule that follows: never estimate an actual dimension from a nominal designation. Look it up.
Treating nominal size as a measurement — the single most common source of dimensional error in fabrication, particularly with pipe, lumber and BSP threads.
This site uses Google Analytics to count visits and see which pages are useful. It sets cookies only if you agree. See the privacy policy.