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 1/64 to 1/32, but a much smaller percentage between 1 47/64 and 1 3/4.ĭrill bit sizes are written as irreducible fractions. 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.

market.ĪNSI 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.įractional-inch drill bit sizes Fractional drill bit set by Craftsmanįractional-inch drill bit sizes are still in common use in the United States and in any factory (around the globe) that makes inch-sized products for the U.S. Metric dimensioning is routinely used for drill bits of all types, although the details of BS 328 apply only to twist drill bits. There are also Renard series sequences of preferred metric 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 the wishes of designers to have drill bit sizes available within at most 10% of an arbitrary hole size. In smaller sizes, bits are available in smaller diameter increments. For example, British Standard BS 328 defines 230 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. The case that holds them in an indexed order (by size), via a graduated series of holes, is called a drill index. Metric drill bit sizes Metric drill set, 1.0–6.0 mm by 0.1 mm, jobber length. Ī comprehensive table for metric, fractional wire and tapping sizes can be found at the drill and tap size chart. The British Standards on replacing gauge size drill bits with metric sizes in the UK was first published in 1959. 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. In the U.S., fractional inch and gauge drill bit sizes are in common use. 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. Drill bits are the cutting tools of drilling machines.