1. n. 
A wellbore that is essentially the same diameter as the bit that was used to drill it. It is common to find well-consolidated sandstones and carbonate rocks that remain gauge after being drilled. For clays, it is common for the hole to slowly enlarge with the passing of time, especially if water-base muds are being used. Bit gauges, rings of defined circumference, are slipped around drill bits to detect and measure wear, which reduces the circumference of the bit during drilling.