Calibre refers to the specific model or design of a watch movement, indicating its unique mechanical configuration and features.
Calibre (European spelling) or caliber (American) refers to the specific movement design used in a watch, identified by a manufacturer's designation number. The term historically referred to the size of a movement (from French, derived from Arabic qālib meaning 'mold'). Today it identifies both the movement's dimensions and its specific design iteration. Rolex Calibre 3235, Patek Philippe Calibre 324, and ETA Calibre 2824-2 are examples that specify exact movement models.
An in-house calibre is designed and manufactured entirely by the watch brand itself—from raw materials through finished movement. An ébauche (or lever blank) is a partially completed movement purchased from specialist movement manufacturers like ETA, Sellita, or Soprod, then finished and adjusted by the watch brand. Many respected brands use modified ébauches. Fully in-house movements command premium prices and are points of prestige, but skilled ébauche-based movements can be equally reliable.
The calibre number is typically printed on the movement itself, visible through an exhibition caseback. It's also documented in the watch's papers, on the swing tag, and in official brand literature. Watch databases like the Caliber Corner and brand-maintained technical documents list calibre specifications. For watches without exhibition casebacks, the reference number can identify the likely calibre—brands publish which movements power which references. A watchmaker can identify the calibre during service.