Drill Size Chart

US drill bits come in three separate sizing systems: fractional, letter, and number. All three describe the same thing, the decimal-inch diameter of the hole the bit cuts, just in different notations. This page is the full fractional reference; the letter and number systems each get their own complete chart, linked below.

Fractional sizes run from 1/64" to 1" in 1/64" steps and cover most general drilling. Number sizes (#1 through #80) cover the smallest diameters and most machine-screw tap drills. Letter sizes (A through Z) fill the gap between fractional 1/4" and 1/2", which is where several common bolt tap drills happen to land.

Every fractional drill size from 1/64" to 1", with its exact decimal-inch and millimeter equivalent. This is the same 1/64" progression used on a tape measure, so a fractional drill's size can always be checked directly against a ruler mark.

Letter drills run from A (0.234", just above 15/64") to Z (0.413", just below 27/64"), filling the range between fractional 1/4" and 1/2" in finer steps than the fractional system offers there. They matter most for tap drills: several common bolt and machine screw tap sizes land on a letter size rather than a clean fraction. See the full letter drill size chart (A to Z) for every decimal and millimeter value.

Number drills run from #1 (0.228", the largest) down to #80 (0.0135", the smallest), covering the fine end of the size range that fractional bits skip over in bigger steps. Most small machine-screw tap drills, and any hole finer than 1/16", use a number size. See the full number drill size chart (#1 to #80) for all 80 sizes with decimal and millimeter equivalents.

If you already know the fraction, decimal, or millimeter size you need, this chart tells you which fractional bit to reach for. If a print or tap drill chart specifies a letter or number instead, use the dedicated letter or number chart to convert it to a decimal value you can check with a caliper. For guidance on which sizes to actually buy first, see the drill bit buying guide.