Drill Bit Size Lookup

How it is calculated

This is a lookup, not a formula. Number-gauge and letter-designated drill bits are defined by a table of diameters in inches (ANSI/ASME B94.11M), and the tool holds all of them: #1 to #80 and A to Z, 106 sizes. The diameter in millimetres is calculated as mm = inches × 25.4 and rounded to two decimals, while the inch value is shown to four decimals. Larger numbers mean smaller bits within the number gauge, so #30 is thinner than #10, which is thinner than #1. In the letter series, later letters are larger, so F is bigger than A. Fractional and metric bits are separate series and are not in the list.

Common mistakes

• Assuming a larger gauge number means a larger bit. In the number series it is the opposite: #30 (0.1285 in) is far smaller than #1 (0.2280 in).
• Mixing up the number and letter series. Letters A to Z are all larger than the whole number gauge: A is 0.2340 in, bigger than #1 at 0.2280 in.
• Treating the rounded millimetre value as the exact size. 3.26 mm is rounded from 3.2639, so for a tight fit rely on the four-decimal inch figure.
• Choosing a tap-drill size from this tool alone. It gives the diameter of a given bit, not which bit a given thread needs; use a tap-drill chart for that.
• Reading #80 for a tiny millimetre figure and trusting the rounding. #80 is 0.0135 in, shown as 0.34 mm, so at this end the inch figure matters even more.