Drill bit sizes

s 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 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:

R5 : M2.5, M4, M6, M10, M16, M24
R10 : 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-inch drill bit sizes are still in common use in the United States and in any factory 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 inch through 1 inch in -inch increments. For Morse taper-shank drill bits, the standard continues in -inch increments up to inches, then -inch increments up to inches, -inch increments up to 3 inches, -inch increments up to inches, and a single -inch increment to 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 to, but a much smaller percentage between and.
Drill bit sizes are written as irreducible fractions. So, instead of inches, or inches, the size is noted as inches.
Below is a chart providing the decimal-fraction equivalents that are most relevant to fractional-inch drill bit sizes.

Decimal-fraction equivalents

fraction	in	mm
	-	-
	-	-
	-	-
	-	-
	-	-
	-	-
	-	-
	-	-
	-	-
	-	-
	-	-
	-	-
	-	-
	-	-
	-	-
	-	-

fraction	in	mm
	-	-
	-	-
	-	-
	-	-
	-	-
	-	-
	-	-
	-	-
	-	-
	-	-
	-	-
	-	-
	-	-
	-	-
	-	-
	-	-

fraction	in	mm
	-	-
	-	-
	-	-
	-	-
	-	-
	-	-
	-	-
	-	-
	-	-
	-	-
	-	-
	-	-
	-	-
	-	-
	-	-
	-	-

fraction	in	mm
	-	-
	-	-
	-	-
	-	-
	-	-
	-	-
	-	-
	-	-
	-	-
	-	-
	-	-
	-	-
	-	-
	-	-
	-	-
	-	-

Number and letter gauge drill bit sizes

Number drill bit gauge sizes range from size 80 to size 1 followed by letter gauge size A to size Z. 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. 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.

Drill bit conversion table

gauge	in	mm
104		-
103		-
102		-
101		-
100		-
99		-
98		-
97		-
96		-
95		-
94		-
93		-
92		-
91		-
90		-
89		-
88		-
87		-
86		-
85		-
84		-
83		-
82		-
81		-
80		-
79		-

gauge	in	mm
78		-
77		-
76		-
75		-
74		-
73		-
72		-
71		-
70		-
69		-
68		-
67		-
66		-
65		-
64		-
63		-
62		-
61		-
60		-
59		-
58		-
57		-
56		-
55		-
54		-
53		-

gauge	in	mm
52		-
51		-
50		-
49		-
48		-
47		-
46		-
45		-
44		-
43		-
42		-
41		-
40		-
39		-
38		-
37		-
36		-
35		-
34		-
33		-
32		-
31		-
30		-
29		-
28		-
27		-

gauge	in	mm
26		-
25		-
24		-
23		-
22		-
21		-
20		-
19		-
18		-
17		-
16		-
15		-
14		-
13		-
12		-
11		-
10		-
9		-
8		-
7		-
6		-
5		-
4		-
3		-
2		-
1		-

gauge	in	mm
A		-
B		-
C		-
D		-
E		-
F		-
G		-
H		-
I		-
J		-
K		-
L		-
M		-
N		-
O		-
P		-
Q		-
R		-
S		-
T		-
U		-
V		-
W		-
X		-
Y		-
Z		-