Inch (in)
The inch (in) is the base small unit of the imperial and US customary length system.
- Origin
- Historically derived from thumb and barleycorn measures, it has been defined internationally since 1959 as exactly 25.4 mm.
- Where it is used
- Standard on US and UK drawings, pipe and fastener sizes, screen dimensions and imported product data.
- Relation to SI
- 1 in = 0.0254 m exactly, by international agreement.