HesapHanem

Drill Bit Size Lookup

Pick a drill bit designation to see its diameter.

Result

0.1285 in
3.26 mm
#30

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.

Worked example

Choose the #30 number-gauge bit. The table gives a diameter of 0.1285 in. Converting to millimetres: 0.1285 × 25.4 = 3.2639, which the tool shows as 3.26 mm. Compare it with #10, which is 0.1935 in or 4.91 mm, and #1 at 0.2280 in or 5.79 mm. So within the number gauge, moving from #30 to #1 grows the bit from about 3.26 mm to 5.79 mm. The tool also repeats the designation (#30).

Try this example →

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.

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. This tool lists all 106 of them, #1 to #80 and A to Z.

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.

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