Drill Bit Size Lookup

About this calculator

Beyond the familiar fractional-inch bits, machinists and toolmakers rely on two finer-grained sizing systems standardized in ANSI/ASME B94.11M: number-gauge bits running from No. 80 (about 0.0135 in, the smallest) up to No. 1 (0.228 in), and letter-designated bits from A (0.234 in) to Z (0.413 in), filling in the gaps the fractional and number systems leave.

These sizes matter most for tap drilling — picking the correct pilot-hole diameter before cutting internal threads — where being off by even a few thousandths of an inch produces threads that are too loose to hold or too tight to cut cleanly. A quick lookup by designation, rather than a mental conversion, is the standard shop-floor way to confirm a bit's exact diameter in both inches and millimetres before drilling.