The gauge (size) of wire is the measure of its diameter. The gauge is notated numerically; so the bigger the number the thinner the wire. An exception, Music Wire is numbered in increasing order.

There are a variety of rating systems depending on type of wire (ferrous, non-ferrous), country standards and/or industry standards. For example – 16-ga. Stainless and 16-ga. Brass are not the same thickness, and 14-ga. Wire may not be the same thickness depending on where the supplier is located so make sure you double check with the supplier.

American Wire Gauge (A.W.G.) also known as Brown & Sharpe(B.S.).
       Used for copper, aluminum,gold, silver, etc.
British or Imperial Standard Wire Gauge (S.W.G.).

       Used in Great Britian and Canada
US Steel Wire also known as Washburn & Moen (W & M) and the Roebling Gauge
       Used for steel.
Birmingham or Stubs IronGauge
      Used for iron.

This can lead to some confusion when buying and/or ordering wire or pre-cut rings. A good way to make sure you are getting the right size wire is to know the decimal that corresponds to the gauge you want. Be aware that the wire manufacturer has a plus or minus tolerance when drawing the wire. If you get wire that is 0.0001 off don’t get upset, unless you are working with 40-gauge wire!

A Vernier gauge caliper or a sheet and wire gauge can be bought through most hardware stores and will come in very handy when shopping around or when you forget to label your .030, .032 and .035 wire.

The following chart gives decimal inches and millimeter equivalents for the most common standards.