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

*N*is an integer from 10 through 29:

*N*· 0.1 mm*N*· 0.1 + 0.05 mm

*N*is an integer from 30 through 139:

*N*· 0.1 mm

*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

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.

[edit] Fractional-inch drill bit sizes Fractional drill bit set by Craftsman 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.

Fractional inch drill bit sizes are still in common use in the US. In the past, they were popular elsewhere, but now have been largely discarded in favour of metric sizes.

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 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. 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.

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 favour of metric sizes.

Jobber-length drills are the most common type of drill. The length of the flutes is either four or five times the diameter of the drill, depending on the manufacturer. So a 1⁄2 in (12.7 mm) diameter drill will be able to drill a hole either 2 or 21⁄2 in (50.8 or 63.5 mm) deep, again depending on the manufacturer.[

*citation needed*]

Long series drill bits 11/32 inch drills - long-series morse, plain morse, jobber The image shows a

**long series drill**compared to its diametric equivalents, all are 11⁄32 inches (8.7 mm) 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, Numbers 1 (bottom) through to 6 (top) Center drills are available with two different included angles; 60 degrees is the standard for drilling centre holes (for example for subsequent centre support in the lathe), but 90 degrees is also common and used when locating holes prior to drilling with twist drills.

**Size Designation**

**Drill Diameter**

[inches

[

**(**mm

**)]**5/0 0.010 in (0.254 mm) 4/0 0.015 in (0.381 mm) 3/0 0.020 in (0.508 mm) 2/0 0.025 in (0.635 mm) 0 1⁄32 in (0.794 mm) 1 3⁄64 in (1.191 mm) 2 5⁄64 in (1.984 mm) 3 7⁄64 in (2.778 mm) 4 1⁄8 in (3.175 mm) 4½ 9⁄64 in (3.572 mm) 5 3⁄16 in (4.763 mm) 6 7⁄32 in (5.556 mm) 7 1⁄4 in (6.350 mm) 8 5⁄16 in (7.938 mm)

**Gauge**[

**inches**(

**mm**)] BS1 1⁄8 in (3.175 mm) BS2 3⁄16 in (4.763 mm) BS3 1⁄4 in (6.350 mm) BS4 5⁄16 in (7.938 mm) BS5 7⁄16 in (11.113 mm) BS5A 1⁄2 in (12.700 mm) BS6 5⁄8 in (15.875 mm) BS7 3⁄4 in (19.050 mm)

Source: http://en.wikipedia.org/wiki/Drill_bit_sizes