The diameter of a drill bit, usually given in inches. Variations in hole size around (above or below) this value are important for a number of reasons. Under gauge well sections (i.e. hole widths less than the bit size) may indicate mudcake build up, sloughing shales, squeeze and hole collapse. Holes larger than the nominal bit size may mark ovalisation, borehole breakout, washout and generally rugose sections.
Proposed bit size through a section is an important consideration in the predicted utility of acoustic borehole imaging tools, as large boreholes results in rapid attenuation of the acoustic signal often with little data of geological merit imaged. In dipmeter and image processing if a dipmeter caliper is not available to define hole shape then bit size can be used as a proxy for the hole diameter.
See also calipers.