I was telling a friend that I'm learning to weld and the discussion got to the meaning of gauge, like 10 gauge - 0.1345" thickness. He related it to shotguns and said, "I think the idea has something to do with how many of something will fit in a standard container. So if 20 of them fit in the container, they’re one size, but if only 10 fit in there, they’re a lot bigger."
Seemed logical for shotguns but not sheet metal so I decided to look it up online -- changing my spelling to "gage" as a result. Then I got really confused:
"Dictionaries (at least in the US) seem to consider "gage" to be an alternate spelling of "gauge". But common usage suggests "gage" refers to sheet metal thickness, gage blocks, and most concrete measurement systems and devices. "Gauge" seems to refer to pressure gauges and level gauges. Comparing Machinery's Ready Reference, Machinery's Handbook, and the MSC catalog, there is no agreement on "wire gage" versus "wire gauge". Time seems to have corrupted any clear distinction, and things are most likely different in England and other places.
So my particular preference is to only use "gauge" for devices that measure more abstract things like pressure and level.
Gage: An arbitrary assignment of numbers to size, used on sheet, wire, and many other things (tubing, shotgun bore, needles, and so on). Unfortunately there are so many different standards that using gage to specify a material is almost useless, and perhaps even dangerous. If in doubt, use decimals.
For example, in U.S. gage, the standard for sheet metal is based on the weight of the metal, not on the thickness. 16-gage is listed as approximately .0625 inch thick and 40 ounces per square foot (the original standard was based on wrought iron at .2778 pounds per cubic inch; steel has almost entirely superseded wrought iron for sheet use, at .2833 pounds per cubic inch). Smaller numbers refer to greater thickness. There is no formula for converting gage to thickness or weight."
These "definitions" are more confusing:
1. “Gauge refers to the thickness of either the pipe wall for both chainlink and ornamental fences or the thickness of the wire for chainlink mesh. The thicker the pipe or wire wall, the smaller the gauge. For example, 16 gauge pipe is thicker than 17 gauge and 9 gauge wire is thicker than 11 gauge.
In reality, when talking about chain link mesh, GAUGE MEANS NOTHING! Why? Because what is 11 gauge chainlink mesh to one company, may not be the same to another. And, it seems that every year the wire gets thinner but the gauge remains the same.”
2. “Gauge measures the thickness of the shrink film. A higher gauge number would correspond to a thicker film.”
This one was more helpful:
"Sheet metal thickness gages are based on a weight of 41.82 pounds per square foot per inch of thickness. This is known as the Manufacturers' Standard Gage for Sheet Steel, and is primarily used for sheet steel. For materials such as Aluminum and Brass the thicknesses will be different. Thus a 10 gage steel sheet which has a thickness of 0.1345 inch will weigh 41.82*0.1345 = 5.625 pounds per square foot."
If you have a better definition, please let me know. Thanks.