Wire "gauge" is a system of measurement used to measure the diameter of certain electricity-conducting wiring. Other names for it include "American wire gauge" and "standardized wire gauge." There are 40 different gauge sizes, ranging from "0000," the smallest, to 36, the largest. With a little work, wire gauge sizes can be converted to millimeters.
Subtract the number of the wire gauge you want to convert to millimeters, in this case, 20, from 36. This equals 16.
Divide 16 by 39. This equals 0.410256. Thirty-nine represents the amount of "steps" in between the various gauges, or the spaces between the 40 different gauge sizes.
Multiply 92 by the power of 0.410256. This equals 6.3921. Ninety-two is the ratio of the diameters of the 40 different gauge sizes.
Multiply 6.3921 by 0.127 mm, the diameter of 36-gauge wire. That equals 0.8117 mm, which is the diameter of a 20-gauge wire.