So, to answer the title question of “How Steel Gauge Works” is simple: it doesn’t. I’ve never found any real rhyme or reason behind the gauges.
From Wikipedia:
“The thickness of sheet metal is commonly specified by a traditional, non-linear measure known as its gauge. The larger the gauge number, the thinner the metal. Commonly used steel sheet metal ranges from 30 gauge to about 7 gauge. Gauge differs between ferrous (iron based) metals and nonferrous metals such as aluminum or copper; copper thickness, for example is measured in ounces (and represents the thickness of 1 ounce of copper rolled out to an area of 1 square foot).”
Also:
“ASTM states in specification ASTM A480-10a “The use of gage number is discouraged as being an archaic term of limited usefulness not having general agreement on meaning.”.”
OK, so this is another one of our convoluted Imperial systems… almost as bad as shoe sizes.
There IS a method to the madness however… regular sheet steel goes by pounds per sq ft. Starting with 3ga at 10 lbs/sq ft right down to 30 gauge at 0.5 lbs/sq ft.
Each gauge number weighs less per square foot than the last one. Following the chart on Engineering Toolbox:
3ga = 10 lbs (160 oz)
3 to 14 gauge: lose 10 oz per gauge
15 to 16 gauge: lose 5 oz per gauge
17 to 20 gauge: lose 4 oz per gauge
21 to 26 gauge: lose 2 oz per gauge
27 to 30 gauge: lose 1 oz per gauge
This could be rather helpful at the steelyard… if all you have is a tape measure and a scale, you could tell how thick a piece of flat steel is, without calipers. Since 18 gauge steel should weigh 2 lbs/sq ft, a 4’x8’ sheet should weigh 64 lbs. If you weighed it out and it was only 48 lbs, you’d know it was 20 gauge.
Or, if you know the square footage of metal in a drum (about 22 sq ft), you can approximate it’s thickness by weight. A heavy 16ga drum like Wayne uses, at 2.5 lbs per sqft, will weigh around 55 pounds. If it’s only 45 pounds, it’s probably 18 gauge.
I’ve never found any real rhyme or reason behind the gauges. 