Differing Size Sets
Manufacturers produce drill bits in the U.S. that conform to the fractional inch size sets. In most other countries, metric size sets are the norm. All other sets are non-standard or are used to accommodate designs from U.S. manufacturers.
Metric Drill Bit Sizes
The British Standards published the metric size chart for drill bits in 1959. Metric drill bit size charts define the diameter of the bit as a function of its standard metric length. These sizes are typically listed in terms of millimeters (mm), for example: 0.2 mm, 12.0 mm, 32.0 mm, etc.
Fractional Inch Drill Bit Sizes
Previously, fractional inch drill bit sizes were common in the U.S. and elsewhere. Now, outside the U.S., these sizes have largely fallen into disuse in favor of metric sizes. These sizes are listed in terms of fractions of an inch, for example: 1/64, 1 3/4, 3 1/4, etc.
Disadvantages of the Fractional Inch System
The fractional inch size system has several disadvantages compared to the metric size system. First is that the size increment between bits of very small sizes is very large. Second is that there is no standard denominator for labeling (such as 64ths of an inch); drill bits are labeled according to irreducible fractions, which can be confusing. For example, instead of labeling a drill bit as 78/64, the fraction is reduced to 1 7/32 inch. Consumers wishing to compare sizes of drill bits in the fractional inch system must perform the necessary calculations first, which can be both time-consuming and confusing.
Some manufacturers in the U.S. still produce drill bits labeled according to a gauge system similar to the American Wire Gauge (AWG) system, and is mostly applied to twisted drill bits. Drill bits manufactured and labeled according to this number gauge system are labeled 80 (the smallest) to 1 (the largest), and then, increasing in size, A (the smallest) to Z (the largest).
Other Size Systems
Other types of drill bits use different labeling systems; however, these other types of bits are very uncommon, used primarily for industrial purposes or are made-to-order, and so are not of concern to the typical consumer.