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.^{[citation needed]}

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 set, 1.0–6.0 mm by 0.1 mm, jobber length. The case that holds them in an indexed order (by size), via a graduated series of holes, is called a drill index.

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 230 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 the wishes of designers to have drill bit sizes available within at most 10% of an arbitrary hole size.

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.

There are also Renard series sequences of preferred metric drill bits:^[1]

• R5 (factor 1.58) : M2.5, M4, M6, M10, M16, M24
• R10 (factor 1.26): M3, M5, M8, M12, M20, M30

Metric dimensioning is routinely used for drill bits of all types, although the details of BS 328 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

 

Fractional drill bit set by Craftsman

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.

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: 100% for the step from 1/64 to 1/32, but a much smaller percentage between 1 47/64 and 1 3/4.

Drill bit sizes are written 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".)

Decimal-fraction equivalents

 

Decimal-fraction equivalents

fraction in mm 1⁄64 0.015625 0.396875 1⁄32 0.03125 0.79375 3⁄64 0.046875 1.190625 1⁄16 0.0625 1.5875 5⁄64 0.078125 1.984375 3⁄32 0.09375 2.38125 7⁄64 0.109375 2.778125 1⁄8 0.125 3.175 9⁄64 0.140625 3.571875 5⁄32 0.15625 3.96875 11⁄64 0.171875 4.365625 3⁄16 0.1875 4.7625 13⁄64 0.203125 5.159375 7⁄32 0.21875 5.55625 15⁄64 0.234375 5.953125 1⁄4 0.250 6.350

fraction in mm 17⁄64 0.265625 6.746875 9⁄32 0.28125 7.14375 19⁄64 0.296875 7.540625 5⁄16 0.3125 7.9375 21⁄64 0.328125 8.334375 11⁄32 0.34375 8.73125 23⁄64 0.359375 9.128125 3⁄8 0.375 9.525 25⁄64 0.390625 9.921875 13⁄32 0.40625 10.31875 27⁄64 0.421875 10.715625 7⁄16 0.4375 11.1125 29⁄64 0.453125 11.509375 15⁄32 0.46875 11.90625 31⁄64 0.484375 12.303125 1⁄2 0.500 12.700

fraction in mm 33⁄64 0.515625 13.096875 17⁄32 0.53125 13.49375 35⁄64 0.546875 13.890625 9⁄16 0.5625 14.2875 37⁄64 0.578125 14.684375 19⁄32 0.59375 15.08125 39⁄64 0.609375 15.478125 5⁄8 0.625 15.875 41⁄64 0.640625 16.271875 21⁄32 0.65625 16.66875 43⁄64 0.671875 17.065625 11⁄16 0.6875 17.4625 45⁄64 0.703125 17.859375 23⁄32 0.71875 18.25625 47⁄64 0.734375 18.653125 3⁄4 0.750 19.050

fraction in mm 49⁄64 0.765625 19.446875 25⁄32 0.78125 19.84375 51⁄64 0.796875 20.240625 13⁄16 0.8125 20.6375 53⁄64 0.828125 21.034375 27⁄32 0.84375 21.43125 55⁄64 0.859375 21.828125 7⁄8 0.875 22.225 57⁄64 0.890625 22.621875 29⁄32 0.90625 23.01875 59⁄64 0.921875 23.415625 15⁄16 0.9375 23.8125 61⁄64 0.953125 24.209375 31⁄32 0.96875 24.60625 63⁄64 0.984375 25.003125 1 1.000 25.400

Number and letter gauge drill bit sizes

 

A #80 drill bit

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 encompasses the sizes for which twist drill bits are most often used.

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.^[2] The accompanying graph illustrates the change in diameter with change in gauge, as well as the reduction in step size as the gauge size decreases. Each step along the horizontal axis is one gauge size.

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.^{[citation needed]}

 

Drill bit conversion table

 

click to see a ruler comparing millimeters to fractions of an inch

See also: Drill and tap size chart

gauge in mm 104 0.0031 0.079 103 0.0035 0.089 102 0.0039 0.099 101 0.0043 0.109 100 0.0047 0.119 99 0.0051 0.130 98 0.0055 0.140 97 0.0059 0.150 96 0.0063 0.160 95 0.0067 0.170 94 0.0071 0.180 93 0.0075 0.191 92 0.0079 0.201 91 0.0083 0.211 90 0.0087 0.221 89 0.0091 0.231 88 0.0095 0.241 87 0.010 0.254 86 0.0105 0.267 85 0.011 0.279 84 0.0115 0.292 83 0.012 0.305 82 0.0125 0.318 81 0.013 0.330 80 0.0135 0.343 79 0.0145 0.368

gauge in mm 78 0.016 0.406 77 0.018 0.457 76 0.020 0.508 75 0.021 0.533 74 0.0225 0.572 73 0.024 0.610 72 0.025 0.635 71 0.026 0.660 70 0.028 0.711 69 0.0292 0.742 68 0.031 0.787 67 0.032 0.813 66 0.033 0.838 65 0.035 0.889 64 0.036 0.914 63 0.037 0.940 62 0.038 0.965 61 0.039 0.991 60 0.040 1.016 59 0.041 1.041 58 0.042 1.067 57 0.043 1.092 56 0.0465 1.181 55 0.052 1.321 54 0.055 1.397 53 0.0595 1.511

gauge in mm 52 0.0635 1.613 51 0.067 1.702 50 0.070 1.778 49 0.073 1.854 48 0.076 1.930 47 0.0785 1.994 46 0.081 2.057 45 0.082 2.083 44 0.086 2.184 43 0.089 2.261 42 0.0935 2.375 41 0.096 2.438 40 0.098 2.489 39 0.0995 2.527 38 0.1015 2.578 37 0.104 2.642 36 0.1065 2.705 35 0.110 2.794 34 0.111 2.819 33 0.113 2.870 32 0.116 2.946 31 0.120 3.048 30 0.1285 3.264 29 0.136 3.454 28 0.1405 3.569 27 0.144 3.658

gauge in mm 26 0.147 3.734 25 0.1495 3.797 24 0.152 3.861 23 0.154 3.912 22 0.157 3.988 21 0.159 4.039 20 0.161 4.089 19 0.166 4.216 18 0.1695 4.305 17 0.173 4.394 16 0.177 4.496 15 0.180 4.572 14 0.182 4.623 13 0.185 4.699 12 0.189 4.801 11 0.191 4.851 10 0.1935 4.915 9 0.196 4.978 8 0.199 5.055 7 0.201 5.105 6 0.204 5.182 5 0.2055 5.220 4 0.209 5.309 3 0.213 5.410 2 0.221 5.613 1 0.228 5.791

gauge in mm A 0.234 5.944 B 0.238 6.045 C 0.242 6.147 D 0.246 6.248 E 0.250 6.350 F 0.257 6.528 G 0.261 6.629 H 0.266 6.756 I 0.272 6.909 J 0.277 7.036 K 0.281 7.137 L 0.290 7.366 M 0.295 7.493 N 0.302 7.671 O 0.316 8.026 P 0.323 8.204 Q 0.332 8.433 R 0.339 8.611 S 0.348 8.839 T 0.358 9.093 U 0.368 9.347 V 0.377 9.576 W 0.386 9.804 X 0.397 10.08 Y 0.404 10.26 Z 0.413 10.49

Screw-machine-length drill

The shortest standard-length drills (that is, lowest length-to-diameter ratio) are screw-machine-length drills (sometimes abbreviated S/M). They are named for their use in screw machines. Their shorter flute length and shorter overall length compared to a standard jobber bit results in a more rigid drill bit, reducing deflection and breakage. They are rarely available in retail hardware stores or home centers.

Jobber-length drill

Jobber-length drills are the most commonly found type of drill. The length of the flutes is between 9 and 14 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 9 times the diameter in length. A 1/8 in (3.175 mm) diameter drill can drill a hole 1+5/8 in (41.275 mm) deep, since it is 13 times the diameter in flute length.^[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. Thus, at the time that the name of jobber-length drill bits became common, it reflected the same concept that names like general-purpose and multipurpose reflect.^{[citation needed]}

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 (3.175 mm), they are available in fixed lengths such as 6, 8, 12 or 18 inches (152, 203, 305 or 457 mm) rather than the progressive lengths of jobber drills.

 

11/32 inch drills: long-series Morse, plain Morse, jobber

The image shows a long-series drill compared to its diametric equivalents, all are 11⁄32 inch (8.7313 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, numbers 1 (bottom) through to 6 (top)

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. Center drills are made specifically for drilling lathe centers, but are often used as spotting drills because of their radial stiffness.

Size designation Drill diameter [inches (mm)] 5/0 0.010 in (0.254 mm) 4/0 0.015 in (0.381 mm) 3/0 0.020 in (0.508 mm) 2/0 0.025 in (0.635 mm) 0 1⁄32 in (0.794 mm) 1 3⁄64 in (1.191 mm) 2 5⁄64 in (1.984 mm) 3 7⁄64 in (2.778 mm) 4 1⁄8 in (3.175 mm) 4½ 9⁄64 in (3.572 mm) 5 3⁄16 in (4.763 mm) 6 7⁄32 in (5.556 mm) 7 1⁄4 in (6.350 mm) 8 5⁄16 in (7.938 mm)

Gauge Body diameter [inches (mm)] BS1 1⁄8 in (3.175 mm) BS2 3⁄16 in (4.763 mm) BS3 1⁄4 in (6.350 mm) BS4 5⁄16 in (7.938 mm) BS5 7⁄16 in (11.113 mm) BS5A 1⁄2 in (12.700 mm) BS6 5⁄8 in (15.875 mm) BS7 3⁄4 in (19.050 mm)

Spotting drill bit sizes

Spotting drills are available in a relatively small range of sizes, both metric and imperial, as their purpose is to provide a precise spot for guiding a standard twist drill. Commonly available sizes are 1/8", 1/4", 3/8", 1/2", 5/8", 3/4", 4 mm, 6 mm, 8 mm, 10 mm, 12 mm, 14 mm, 16 mm and 18 mm. The drills are most ordinarily available with either 90° or 120° included angle points.

See also

• Drill and tap size chart

References

1. Dasarathi-Cadem (2 February 2017). "Preferred sizes in engineering". CNC etc. Retrieved 6 September 2017.
2. Pöll, J. S. (June 1999). "The story of the gauge". Anaesthesia. 54 (6): 575–581. doi:10.1046/j.1365-2044.1999.00895.x. ISSN 0003-2409. PMID 10403873. S2CID 41519791.
3. "Jobber Length Twist Drill Sizes".