Reference dimension

A reference dimension is a dimension on an engineering drawing provided for information only. Reference dimensions are provided for a variety of reasons and are often an accumulation of other dimensions that are defined elsewhere (e.g. on the drawing or other related documentation). These dimensions may also be used for convenience to identify a single dimension that is specified elsewhere (e.g. on a different drawing sheet).

Reference dimensions are not intended to be used directly to define the geometry of an object. Reference dimensions do not normally govern manufacturing operations (such as machining) in any way and, therefore, do not typically include a dimensional tolerance (though a tolerance may be provided if such information is deemed helpful). Consequently, reference dimensions are also not subject to dimensional inspection under normal circumstances.

Reference dimensions are commonly used in CAD software along with constraints that usually denote the opposite: mandatory dimensions to be precisely followed.