American Wire Gauge is a standard used in the United States since 1857 for copper, aluminium, gold, silver, etc. It is also known as Brown & Sharpe wire gauge. The more is the gauge number, the smaller is the wire diameter. Wires thicker than gauge 0 are denoted as 00 (or 2/0), 000 (or 3/0), etc.
Washburn & Moen Steel Wire Gauge is used in the United States for steel. It is also known as US Steel Wire or Roebling Gauge.
British Standard Wire Gauge is no longer popular, but is still used for measuring guitar strings. It is also known as Imperial Wire Gauge or British Standard Gauge. SVG only defines gauges from 7/0 to 50.
Birmingham Wire Gauge is now obsolete. Its sizes are not geometrically or arithmetically progressive, and, consequently, bear no definite relation to each other. B.W.G defines gauges from 5/0 to 36.
IEC 60228 is an international standard that among other things defines a set of standard wires. Wire sizes in this standard are denoted by their cross-section area expressed in mm². GOST 22483-2012 is a slightly modified version of IEC 60228 that is used in Russia, Belarus, Kyrgyzstan and Uzbekistan. It adds several smaller wire sizes to the international standard.
If a section area or diameter of the wire does not match a gauge, the nearest one is used and the difference of the area is shown as a percentage.