Drill bit sizes

In nearly all other countries, metric drill bit sizes are most common, and all others are anachronisms or are reserved for dealing with designs from the US.

Sets are not commonly available in smaller size increments, except for drill bits below 1 mm diameter.

Fractional-inch drill bit sizes are still in common use in the United States and in any factory (around the globe) that makes inch-sized products for the U.S. market.

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"].

The gauge-to-diameter ratio is not defined by a formula; it is based on—but is not identical to—the Stubs Steel Wire Gauge, which originated in Britain during the 19th century.

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 supplanted by metric sizes.

Other countries that formerly used the number series have for the most part also abandoned these in favour of metric sizes.

[3] The term jobber refers to a wholesale distributor—a person or company that buys from manufacturers and sells to retailers.

[citation needed] Manufacturers producing drill bits "for the trade" (as opposed to for specialized machining applications with particular length and design requirements) made ones of medium length suitable for a wide variety of jobs, because that was the type most desirable for general distribution.

[citation needed] Extended-reach or long-series drills are commonly called aircraft-length from their original use in manufacturing riveted aluminum aircraft.

The image shows a long-series drill compared to its diametric equivalents, all are 11⁄32 inch (8.7313 mm) in diameter.