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 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.
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
See the rest of the chart →. US number and letter gauge drill bit sizes
Number drill bit gauge sizes are analogous to, but different from, American wire gauge..
Number gauge is routinely used from size 80 to size 1 followed by letter gauge size A
to size Z. 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 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
Screw-machine-length drill The shortest standard-length drills 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.
Jobber-length drill 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 diameter drill will be able to drill a hole
4 1⁄2 in deep since it is nine times the diameter in length. A 1⁄8 in diameter drill
can drill a hole 1 5⁄8 in 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 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, but 90 degrees is also common and used when locating holes
prior to drilling with twist drills. See also
Drill and tap size chart