It is my understanding that a mil is a one thousandth of an inch (1mil = 0.001”). Also, a mil is 25.4 microns, aka micrometers. Then, it is my understanding that Gauge is 100x mil (1mil-100Ga).

I am looking for some copper sheets for a project, and I am confused. This Amazon listing: enter image description here

Lists 0.0216" (inch) as a 24Ga (Gauge). How does that work? Am I reading the units wrong? 0.0216" = 21.6Mil = 2160Ga ? Is that right?

  • 5
    $\begingroup$ "Gauge" is not a unit at all. The meaning of "24 gauge" here is closer to "size number 24" than "24 gauges of thickness." $\endgroup$
    – Air
    Mar 22, 2016 at 19:15

2 Answers 2


Gage (gauge) thickness are somewhat arbitrary. Take for example the table below, excerpted from the AISC Steel Construction Manual, 14th Ed.:

sheet metal gages

You can see that gage thicknesses vary depending on material and coating but are approximately the same for a given gage number.

This PDF references the following for different material gages:

Note that the B&S and A.W.G. link shows a rather complicated formula for determining wire gauge:

$$ d_n = 0.005\mbox{inch} * 92^\frac{36-n}{39} = 0.127\mbox{mm} * 92^\frac{36-n}{39} $$

where $n$ is the gauge number for gauges number 32 to 0.


Most gauge systems like SWG aren't linear and are based on a volumetric or cross sectional area system of some sort. In the case of wire gauges it is usually derived from the number of drawing operations required to produce a particular size of wire, so the gauge number is not proportional to the diameter but rather inversely proportional to the cross sectional area and thus mass per unit length.

Typically the gauge number, n comes from 1/n times some reference quantity.

Similarly the gauge of shot is the number of spheres which can be made from a pound of lead so it is a measure of the inverse of volume even though it is used to indicate a diameter.

So if you want to relate gauge to diameter or thickness the easiest solution is to look it up in a table.

Note also that a large gauge number corresponds to a thinner wire.

  • $\begingroup$ What does SWG stand for? $\endgroup$
    – JPhi1618
    Mar 22, 2016 at 20:47
  • $\begingroup$ Standard Wire Gauge. (It's a UK standard) $\endgroup$ Mar 22, 2016 at 21:29

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