Number & Letter Gauge Drill Bits

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Number drill bit gauge sizes range from size 80 (the smallest) to size 1 (the largest) followed by letter gauge size A (the smallest) to size Z (the largest). 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 encompass the sizes for which twist drill bits are most often used.

The gauge-to-diameter ratio is not defined by a formula, but is instead based on, but is not identical to, the Stubs Steel Wire Gauge, which originated in Britain during the 19th century. The accompanying graph, with each step along the horizontal axis being one gauge size, illustrate the change in diameter with change in gauge, as well as the reduction in step size as the gauge size decreases.

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letter gauge drill bits are still in common use in the U.S. and to a lesser extent the UK, where they have largely been superseded by metric sizes. Other countries that formerly used the number series have for the most part also ...
... abandoned these in favour of metric sizes.

Screw-machine-length drill
The shortest standard-length drills (that is, lowest length-to-diameter ratio) are screw-machine-length drills (sometimes abbreviated S/M). 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 (not in hardware stores or home centers).

Jobber-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 1/2 in (12.7 mm) diameter drill will be able to drill a hole 4 1/2 in (114.3 mm) deep, since it is 9 times the diameter in length. A 1/8 in (3.2 mm) diameter drill can drill a hole 1 5/8 in (41.3 mm) deep, since it is 13 times the diameter in flute length.

Aircraft-length drill[edit]
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.