Drill bit sizes
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 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 mmN · 0.1 + 0.02 mmN · 0.1 + 0.05 mmN · 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 mmN · 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 mmM · 1 + 0.25 mmM · 1 + 0.5 mmM · 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
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
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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 | - |
Screw-machine-length drill
The shortest standard-length drills are screw-machine-length drills. They are named for their use in screw machines. Their shorter flute length and shorter overall length compared to a standard jobber bit results in a more rigid drill bit, reducing deflection and breakage. They are rarely available in retail hardware stores or home centers.Types of Drill Bits
Twist Drills: Most common, general-purpose drillingBrad Point: Woodworking, precise holesSpade Bits: Fast, rough holes in woodForstner Bits: Clean, flat-bottom holesStep Bits: Multiple diameter holesJobber-length drill
Jobber-length drills are the most commonly found type of drill. The length of the flutes is between 9 and 14 times the diameter of the drill, depending on the drill size. So a diameter drill will be able to drill a hole deep, since it is 9 times the diameter in length. A diameter drill can drill a hole deep, since it is 13 times the diameter in flute length.The term jobber refers to a wholesale distributor—a person or company that buys from manufacturers and sells to retailers. Manufacturers producing drill bits "for the trade" made ones of medium length suitable for a wide variety of jobs, because that was the type most desirable for general distribution. Thus, at the time that the name of jobber-length drill bits became common, it reflected the same concept that names like general-purpose and multipurpose reflect.
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, they are available in fixed lengths such as rather than the progressive lengths of jobber drills.The image shows a long-series drill compared to its diametric equivalents, all are 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. Center drills are made specifically for drilling lathe centers, but are often used as spotting drills because of their radial stiffness.| Size designation | Drill diameter |
| 5/0 | |
| 4/0 | |
| 3/0 | |
| 2/0 | |
| 0 | |
| 1 | |
| 2 | |
| 3 | |
| 4 | |
| 4½ | |
| 5 | |
| 6 | |
| 7 | |
| 8 |
| Gauge | Body diameter |
| BS1 | |
| BS2 | |
| BS3 | |
| BS4 | |
| BS5 | |
| BS5A | |
| BS6 | |
| BS7 |