Drill bit sizes
Drill bits are the cutting tools of drilling machines. They can be made in any size to order, but standards organizations have defined sets of sizes that are produced routinely by drill bit manufacturers and stocked by distributors.
In the U.S., fractional inch and gauge drill bit sizes are in common use. 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. The British Standards on replacing gauge size drill bits with metric sizes in the UK was first published in 1959.
A comprehensive table for metric, fractional wire and tapping sizes can be found at the drill and tap size chart.
Metric drill bit sizes
Metric drill bit sizes define the diameter of the bit in terms of standard metric lengths. Standards organizations define sets of sizes that are conventionally manufactured and stocked. For example, British Standard BS 328 defines sizes from 0.2 mm to 25.0 mm.
From 0.2 through 0.98 mm, sizes are defined as follows, where N is an integer from 2 through 9:
- N · 0.1 mm
- N · 0.1 + 0.02 mm
- N · 0.1 + 0.05 mm
- N · 0.1 + 0.08 mm
From 1.0 through 2.95 mm, sizes are defined as follows, where N is an integer from 10 through 29:
- N · 0.1 mm
- N · 0.1 + 0.05 mm
From 3.0 through 13.9 mm, sizes are defined as follows, where N is an integer from 30 through 139:
- N · 0.1 mm
From 14.0 through 25.0 mm, sizes are defined as follows, where M is an integer from 14 through 25:
- M · 1 mm
- M · 1 + 0.25 mm
- M · 1 + 0.5 mm
- M · 1 + 0.75 mm
In smaller sizes, bits are available in smaller diameter increments. This reflects both the smaller drilled hole diameter tolerance possible on smaller holes, and also the wishes of designers to have drill bit sizes available within at most 10% of an arbitrary size hole.
The price and availability of particular size bits does not change uniformly across the size range. Bits at size increments of 1 mm are most commonly available, and lowest price. Sets of bits in 1 mm increments might be found on a market stall. In 0.5 mm increments, any hardware store. In 0.1 mm increments, any engineers' store. Sets are not commonly available in smaller size increments, except for drill bits below 1 mm diameter. Drill bits of the less routinely used sizes, such as 2.55 mm, would have to be ordered from a specialist drill bit supplier. This subsetting of standard sizes is in contrast to general practice with number gauge drill bits, where it is rare to find a set on the market which does not contain every gauge.
Metric dimensioning is routinely used for drill bits of all types, although the details of BS328 apply only to twist drill bits. For example, a set of forstner bits may contain 10, 15, 20, 25 and 30 mm diameter cutters.
Fractional-inch drill bit sizes
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.
Decimal-fraction equivalents: 0 to 1 by 64ths
US number and letter gauge drill bit sizes 
Number gauge is routinely used from size 80 (the smallest) to size 1 (the largest) followed by letter gauge size A (the smallest) to size Z (the largest). Number gauge is actually defined at least down to size 97, but these smaller sizes are rarely encountered.
Number and letter gauge drill bits are almost always twist drill bits because the gauge covers a size range across which the twist drill bit is the most commonly used.
The gauge-to-diameter conversion does not follow a set formula, but rather was defined as a useful and practical measure. The graph shows how gauge diameters change with gauge. Each step along the horizontal axis is one gauge size. The step size between adjacent gauges is smaller for smaller gauges. This is appropriate, because the tolerance of the diameter of drilled holes is closer for smaller drill bits. The increment from one gauge to the next for a number 92 drill bit at 0.2 mm diameter is just 5%, compared to 10% for standard metric sizes. Number and letter gauge drill bits are still in common use in the U.S. In the past, they were popular elsewhere, but now have been largely discarded in favor of metric sizes.
The US number and letter size drills are sized as needed to provide proper clearance holes for screws and bolts according to ASME B18.2.8. There are three fit classes for clearance holes: close, normal, and loose. Some of the clearances required are not in increments of 1/64 or 1/32. This necessitates ranges of drill sizes in between the fractional sizes, especially in the smaller diameter numbered screw sizes.
Drill Bit Conversion Table
The shortest standard-length drills (that is, lowest length-to-diameter ratio) are screw-machine-length drills. They get their name from their most common application: use in screw machines. Given the industrial nature of most demand for screw-machine-length drills, they are generally sold only by metalworking supply businesses (not in hardware stores or home centers).
Jobber-length drills are the most common type of drill. The length of the flutes is between nine and fourteen times the diameter of the drill, depending on the drill size. So a 1⁄2 in (12.7 mm) diameter drill will be able to drill a hole 4 1⁄2 in (114.3 mm) deep since it is nine times the diameter in length. A 1⁄8 in (3.2 mm) diameter drill can drill a hole 1 5⁄8 in (41.3 mm) deep since it is thirteen times the diameter in flute length
Aircraft length drill
Extended reach or long series drills are commonly called Aircraft length from their original use in manufacturing riveted aluminum aircraft. For bits thicker than a minimum size such as 1/8 inch, they are available in fixed lengths such as 6, 8, 12, or 18 inches rather than the progressive lengths of Jobber drills.
The image shows a long series drill compared to its diametric equivalents, all are 11⁄32 inch (8.7 mm) in diameter. The equivalent morse taper drill shown in the middle is of the usual length for a taper shank drill. The lower drill bit is the jobber or parallel shank equivalent.
Center drill bit sizes
Center drills are available with two different included angles; 60 degrees is the standard for drilling centre holes (for example for subsequent centre support in the lathe), but 90 degrees is also common and used when locating holes prior to drilling with twist drills.