This is appropriate, because the tolerance of the diameter of drilled holes is closer for smaller drill bits. The step size between adjacent gauges is smaller for smaller gauges. Each step along the horizontal axis is one gauge size. The graph shows how gauge diameters change with gauge.

The gauge-to-diameter conversion does not follow a set formula, but rather was defined as a useful and practical measure. 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. Number and letter gauge drill bits are almost always twist drill bits. 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 gauge is actually defined at least down to size 97, but these smaller sizes are rarely encountered. 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 drill bit gauge sizes are analogous to, but different from, American wire gauge. Decimal-fraction equivalents: 0 to 1 by 64ths In the past, they were popular elsewhere, but now have been largely discarded in favour of metric sizes. 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'.)įractional inch drill bit sizes are still in common use in the US. 75 are shown to thousandths, which is how machinists usually think about them. This can lead to confusion and mistakes unless great care is taken.īelow 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). So, instead of 78/64 inch, or 1 14/64 inch, the size is always written as 1 7/32 inch. Rather than an integral number of 64ths of an inch, drill bit sizes are written down as irreducible fractions. The implication is that number gauge drill bits have to be used to bridge the gaps.Īnother disadvantage is the convention in labelling the bits. 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. 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. Fractional-inch drill bit sizesĪNSI 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 example, a set of forstner bits may contain 10, 15, 20, 25 and 30 mm diameter cutters. Metric dimensioning is routinely used for drill bits of all types, although the details of BS328 apply only to twist drill bits. 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. Drill bits of the less routinely used sizes, such as 2.55 mm, would have to be ordered from a specialist drill bit supplier. Sets are not commonly available in smaller size increments, except for drill bits below 1 mm diameter. In 0.1 mm increments, any engineers' store. In 0.5 mm increments, any hardware store. Sets of bits in 1 mm increments might be found on a market stall. Bits at size increments of 1 mm are most commonly available, and lowest price. The price and availability of particular size bits does not change uniformly across the size range. 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. In smaller sizes, bits are available in smaller diameter increments. For example, British Standard BS 328 defines sizes from 0.2 mm to 25.0 mm.įrom 0.2 through 0.98 mm, sizes are defined as follows, where N is an integer from 2 through 9:įrom 1.0 through 2.95 mm, sizes are defined as follows, where N is an integer from 10 through 29:įrom 3.0 through 13.9 mm, sizes are defined as follows, where N is an integer from 30 through 139:įrom 14.0 through 25.0 mm, sizes are defined as follows, where M is an integer from 14 through 25: Standards organizations define sets of sizes that are conventionally manufactured and stocked.

Metric drill bit sizes define the diameter of the bit in terms of standard metric lengths. Metric drill set, 1.0-6.0 mm by 0.1 mm, jobber length

Drill bit size chart