See: https://en.wikipedia.org/wiki/American_wire_gauge

Wire sizes and the corresponding numbered drill sizes make sense because a very wide range of commonly used sizes can be defined with just a single or two digit number. A #36 drill is defined as 0.005" or 5 mils. Every three wire sizes is double or half the cross-sectional area, which is useful for determining wire size according to current carrying capacity. So a #10 AWG is good for 30 amps, #13 is 15 amps, and #16 is 7.5 amps.

For wire, each increment of size corresponds to about 12% difference in diameter, For drills, each increment of size corresponds to about 1.5% to 5% difference in diameter, which is useful for making holes just a bit larger or smaller. It is not so obvious what the next standard metric size might be, although unlike number drill sizes, there is a formula.

From 0.2 through 0.98 mm, sizes are defined as follows, where

From 1.0 through 2.95 mm, sizes are defined as follows, where

From 3.0 through 13.9 mm, sizes are defined as follows, where

From 14.0 through 25.0 mm, sizes are defined as follows, where

Wire sizes and the corresponding numbered drill sizes make sense because a very wide range of commonly used sizes can be defined with just a single or two digit number. A #36 drill is defined as 0.005" or 5 mils. Every three wire sizes is double or half the cross-sectional area, which is useful for determining wire size according to current carrying capacity. So a #10 AWG is good for 30 amps, #13 is 15 amps, and #16 is 7.5 amps.

For wire, each increment of size corresponds to about 12% difference in diameter, For drills, each increment of size corresponds to about 1.5% to 5% difference in diameter, which is useful for making holes just a bit larger or smaller. It is not so obvious what the next standard metric size might be, although unlike number drill sizes, there is a formula.

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

From 1.0 through 2.95 mm, sizes are defined as follows, where

*N*is an integer from 10 through 29:*N*· 0.1 mm*N*· 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 mm*M*· 1 + 0.25 mm*M*· 1 + 0.5 mm*M*· 1 + 0.75 mm

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