Drill bitDrill bits are cutting tools used to create cylindrical holes. Bits are held in a tool called a drill, which rotates them and provides torque and axial force to create the hole. Specialized bits are also available for non-cylindrical-shaped holes....
s are the cutting tools of
drilling machinesA drill is a tool with a rotating drill bit used for drilling holes in various materials. The drill bit is gripped by a chuck at one end of the drill and rotated while pressed against the target material...
. They can be made in any size to order, but
standards organizationA standards organization, standards body, standards development organization or SDO is any entity whose primary activities are developing, coordinating, promulgating, revising, amending, reissuing, interpreting, or otherwise maintaining standards that address the interests of a wide base of users...
s have defined sets of sizes that are produced routinely by drill bit manufacturers and stocked by distributors.
In the
U.S.The United States of America is a federal constitutional republic comprising fifty states and a federal district...
, 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.
Drill bitDrill bits are cutting tools used to create cylindrical holes. Bits are held in a tool called a drill, which rotates them and provides torque and axial force to create the hole. Specialized bits are also available for non-cylindrical-shaped holes....
s are the cutting tools of
drilling machinesA drill is a tool with a rotating drill bit used for drilling holes in various materials. The drill bit is gripped by a chuck at one end of the drill and rotated while pressed against the target material...
. They can be made in any size to order, but
standards organizationA standards organization, standards body, standards development organization or SDO is any entity whose primary activities are developing, coordinating, promulgating, revising, amending, reissuing, interpreting, or otherwise maintaining standards that address the interests of a wide base of users...
s have defined sets of sizes that are produced routinely by drill bit manufacturers and stocked by distributors.
In the
U.S.The United States of America is a federal constitutional republic comprising fifty states and a federal district...
, 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 StandardsBritish Standards are produced by BSI Group which is incorporated under a Royal Charter and is formally designated as the National Standards Body for the UK...
on replacing gauge size drill bits with metric sizes in the
UKThe United Kingdom of Great Britain and Northern Ireland is a sovereign state located off the northwestern coast of continental Europe. It is an island country, spanning an archipelago including Great Britain, the northeastern part of Ireland, and many small islands...
was first published in 1959.
A comprehensive table for metric, fractional wire and tapping sizes can be found at the
drill and tap size chartBelow is a comprehensive drill and tap size chart for all drills, imperial and metric, and taps up to 36.5 mm. A version is available as a quick reference.-Chart:...
.
Metric drill bit sizes
Metric drill bit sizes define the diameter of the bit in terms of standard
metricThe metric system is an international decimalised system of measurement, first adopted by France in 1791, that is the common system of measuring units used by most of the world. It exists in several variations, with different choices of fundamental units, though the choice of base units does not...
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
- N · 0.1 mm,
- N · 0.1 + 0.02 mm,
- N · 0.1 + 0.05 mm, and
- N · 0.1 + 0.08 mm, with N an integer from 2 through 9.
From 1.0 through 2.95 mm, sizes are defined as
- N · 0.1 mm and
- N · 0.1 + 0.05 mm, with N an integer from 10 through 29.
From 3.0 through 13.9 mm, sizes are defined as
- N · 0.1 mm, with N an integer from 30 through 139.
From 14.0 through 25.0 mm, sizes are defined as
- M · 1 mm,
- M · 1 + 0.25 mm,
- M · 1 + 0.5 mm, and
- M · 1 + 0.75 mm, with M an integer from 14 through 25.
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
Fractional inch drill bit sizes are still in common use in the US. In the past, they were popular elsewhere, but now have been largely discarded in favour of metric 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 1 3/4 inch, then 1/32 inch increments up to 2 1/4 inch, 1/16 inch increments up to 3 inches, 1/8 inch increments up to 3 1/4 inches, and a single 1/4 inch increment to 3 1/2 inches.
One disadvantage of this scheme of sizing is that the size increment between drill bits is very large for the smaller sizes, 100% for the first step. The implication is that number gauge drill bits have to be used to bridge the gaps.
Another disadvantage is the convention in labelling the bits. Rather than an integral number of 64ths of an inch, drill bit sizes are written down as
irreducible fractionAn irreducible fraction is a vulgar fraction in which the numerator and denominator are smaller than those in any other equivalent vulgar fraction...
s. So, instead of 78/64 inch, or 1 14/64 inch, the size is always written as 1 7/32 inch. This can lead to confusion and mistakes unless great care is taken.
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: 0 to 1 by 64ths
| |
|
|
|
|
|
|
|
0.000 |
| |
|
|
|
|
|
|
|
| |
|
|
|
|
|
1 |
|
.015625 |
| |
|
|
|
|
|
64 |
|
| |
|
|
|
|
1 |
|
|
.03125 |
| |
|
|
|
|
32 |
|
|
| |
|
|
|
|
|
3 |
|
.046875 |
| |
|
|
|
|
|
64 |
|
- See the rest of the chart →.
US number and letter gauge drill bit 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 favour of metric sizes. A complication of using a gauge rather than a standard measurement of length is that the user always has to refer to a table of gauge sizes. For example, to drill a hole which is 10 thou (10 thousandth of an inch = .010) bigger than a 1/8 inch bolt, the user must first work out that 1/8 inch is 125 thou, so the hole size needed is 135 thou, and then must look up in a reference book which gauge drill bit has this diameter - a number 29.
Number drill bit gauge sizes are analogous to, but different from,
American wire gaugeAmerican wire gauge , also known as the Brown & Sharpe wire gauge, is a standardized wire gauge system used since 1857 predominantly in the United States for the diameters of round, solid, nonferrous, electrically conducting wire...
. (See the conversion table below).
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. It happens that as the technology for making small drill bits and drilling small holes has become more available, metric measurements have become the norm.
Number and letter gauge drill bits are almost always twist drill bits. There is no particular reason why the gauge cannot be used to measure bits of other types, but 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.
Drill Bit Conversion Table
| gauge | inches | mm
| | 80 | 0.0135 | 0.343
| | 79 | 0.0145 | 0.368
| | 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
|
|
| gauge | inches | mm
| | 53 | 0.0595 | 1.511
| | 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 | inches | 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 | inches | 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. They get their name from their most common application: use in
screw machineA screw machine is a metalworking machine tool used in the high-volume manufacture of turned components. Screw machines are fundamentally a type of lathe that is specialized for the automated production of small parts...
s. 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 drill
Jobber-length drills are the most common type of drill. The length of the flutes is ten times the diameter of the drill, that is they will drill a hole that is 10D deep.
Long series drill bits
The image shows a
long series drill compared to its diametric equivalents, all are 11/32 inch 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
latheA lathe is a machine tool which spins a block of material to perform various operations such as cutting, sanding, knurling, drilling, or deformation with tools that are applied to the workpiece to create an object which has symmetry about an axis of rotation.Lathes are used in woodturning,...
), but 90 degrees is also common and used when locating holes prior to drilling with twist drills.
| Size Designation | Drill Diameter (inches)
| | 5/0 | .010"
| | 4/0 | .015"
| | 3/0 | .020"
| | 2/0 | .025"
| | 0 | 1/32"
| | 1 | 3/64"
| | 2 | 5/64"
| | 3 | 7/64"
| | 4 | 1/8"
| | 4-1/2 | 9/64"
| | 5 | 3/16"
| | 6 | 7/32"
| | 7 | 1/4"
| | 8 | 5/16"
|
|
| Gauge | inches
| | BS1 | 1/8"
| | BS2 | 3/16"
| | BS3 | 1/4"
| | BS4 | 5/16"
| | BS5 | 7/16"
| | BS5A | 1/2"
| | BS6 | 5/8"
| | BS7 | 3/4"
|
|
External links