Fractional-inch Drill Bit Sizes

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ANSI B94.11M-1979 sets size standards for jobber-length straight-shank twist drill bits from 1/64 inch through 1 inch in 1/64 inch increments. For Morse taper-shank drill bits, the standard continues in 1/64 inch increments up to 1¾ inch, then 1/32 inch increments up to 2¼ inch, 1/16 inch increments up to 3 inches, 1/8 inch increments up to 3¼ inches, and a single 1/4 inch increment to 3½ inches. One aspect of this method of sizing is that the size increment between drill bits becomes larger as bit sizes get smaller sizes: 100% for the step from 1/64th to 1/32nd, but a much smaller percentage between 1 47/64th and 1 3/4.

Drill bit sizes are written down on paper and etched onto bits as irreducible fractions. So, instead of 78/64 inch, or 1 14/64 inch, the size is noted as 1 7/32 inch.

Below is a chart providing the decimal-fraction equivalents that are most relevant to fractional-inch drill bit sizes (that is, 0 to 1 by 64ths). (Decimal places for .25, .5, and .75 are shown to thousandths [.250, .500, .750], which is how machinists usually think about them ["two-fifty", "five hundred", "seven-fifty"]. Machinists ...
... generally truncate the decimals after thousandths; for example, a 27/64" drill bit may be referred to in shop-floor speech as a "four-twenty-one drill".)

Fractional inch drill bit sizes are still in common use in the US. In the past, they were popular elsewhere, but they have now been largely discarded in favour of metric sizes.

Number and letter gauge drill bit sizes

Number drill bit gauge sizes range from size 80 (the smallest) to size 1 (the largest) followed by letter gauge size A (the smallest) to size Z (the largest). Although the ASME B94.11M twist drill standard, for example, lists sizes as small as size 97, sizes smaller than 80 are rarely encountered in practice.

Number and letter sizes are commonly used for twist drill bits rather than other drill forms, as the range encompass the sizes for which twist drill bits are most often used.

The gauge-to-diameter ratio is not defined by a formula, but is instead based on, but is not identical to, the Stubs Steel Wire Gauge, which originated in Britain during the 19th century. The accompanying graph, with each step along the horizontal axis being one gauge size, illustrate the change in diameter with change in gauge, as well as the reduction in step size as the gauge size decreases.

Number and letter gauge drill bits are still in common use in the U.S. and to a lesser extent the UK, where they have largely been superseded by metric sizes. Other countries that formerly used the number series have for the most part also abandoned these in favour of metric sizes.