Drill bit sizes

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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[edit]

Metric drill set, 1.0-6.0 mm by 0.1 mm, jobber length

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:

From 1.0 through 2.95 mm, sizes are defined as follows, where N is an integer from 10 through 29:

From 3.0 through 13.9 mm, sizes are defined as follows, where N is an integer from 30 through 139:

From 14.0 through 25.0 mm, sizes are defined as follows, where M is 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[edit]

Fractional drill bit set by Craftsman

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 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 fractions. 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".)

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.

Decimal-fraction equivalents: 0 to 1 by 64ths[edit]

Click this thumbnail to see the chart as a landscape-oriented image.
        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 [edit]

A #80 drill bit.

Number drill bit gauge sizes are analogous to, but different from, American wire gauge. (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.[citation needed]

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.

Us drill sizes 2.png

Drill Bit Conversion Table[edit]

click to see a ruler comparing millimetres to fractions of an inch
gaugeinmm
800.01350.343
790.01450.368
780.0160.406
770.0180.457
760.0200.508
750.0210.533
740.02250.572
730.0240.610
720.0250.635
710.0260.660
700.0280.711
690.02920.742
680.0310.787
670.0320.813
660.0330.838
650.0350.889
640.0360.914
630.0370.940
620.0380.965
610.0390.991
600.0401.016
590.0411.041
580.0421.067
570.0431.092
560.04651.181
550.0521.321
540.0551.397
gaugeinmm
530.05951.511
520.06351.613
510.0671.702
500.0701.778
490.0731.854
480.0761.930
470.07851.994
460.0812.057
450.0822.083
440.0862.184
430.0892.261
420.09352.375
410.0962.438
400.0982.489
390.09952.527
380.10152.578
370.1042.642
360.10652.705
350.1102.794
340.1112.819
330.1132.870
320.1162.946
310.1203.048
300.12853.264
290.1363.454
280.14053.569
270.1443.658
gaugeinmm
260.1473.734
250.14953.797
240.1523.861
230.1543.912
220.1573.988
210.1594.039
200.1614.089
190.1664.216
180.16954.305
170.1734.394
160.1774.496
150.1804.572
140.1824.623
130.1854.699
120.1894.801
110.1914.851
100.19354.915
90.1964.978
80.1995.055
70.2015.105
60.2045.182
50.20555.220
40.2095.309
30.2135.410
20.2215.613
10.2285.791
--
gaugeinmm
A0.2345.944
B0.2386.045
C0.2426.147
D0.2466.248
E0.2506.350
F0.2576.528
G0.2616.629
H0.2666.756
I0.2726.909
J0.2777.036
K0.2817.137
L0.2907.366
M0.2957.493
N0.3027.671
O0.3168.026
P0.3238.204
Q0.3328.433
R0.3398.611
S0.3488.839
T0.3589.093
U0.3689.347
V0.3779.576
W0.3869.804
X0.39710.08
Y0.40410.26
Z0.41310.49
--

Screw-machine-length drill[edit]

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 drill[edit]

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 12 in (12.7 mm) diameter drill will be able to drill a hole 4 12 in (114.3 mm) deep since it is nine times the diameter in length. A 18 in (3.2 mm) diameter drill can drill a hole 1 58 in (41.3 mm) deep since it is thirteen times the diameter in flute length[citation needed]

Long series drill bits[edit]

11/32 inch drills - long-series morse, plain morse, jobber

The image shows a long series drill compared to its diametric equivalents, all are 1132 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[edit]

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.

Size DesignationDrill Diameter
[inches (mm)]
5/00.010 in (0.254 mm)
4/00.015 in (0.381 mm)
3/00.020 in (0.508 mm)
2/00.025 in (0.635 mm)
0132 in (0.794 mm)
1364 in (1.191 mm)
2564 in (1.984 mm)
3764 in (2.778 mm)
418 in (3.175 mm)
964 in (3.572 mm)
5316 in (4.763 mm)
6732 in (5.556 mm)
714 in (6.350 mm)
8516 in (7.938 mm)
GaugeBody Diameter
[inches (mm)]
BS118 in (3.175 mm)
BS2316 in (4.763 mm)
BS314 in (6.350 mm)
BS4516 in (7.938 mm)
BS5716 in (11.113 mm)
BS5A12 in (12.700 mm)
BS658 in (15.875 mm)
BS734 in (19.050 mm)

See also[edit]