# How steel gauge works

Something interesting, probably most of you guys knew this, I didn’t…

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 chart2 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

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Weight of 1 square foot:

3ga = 160 oz = 10.0 lbs
4ga = 150 oz = 9.4 lbs (9 lbs 6 oz)
5ga = 140 oz = 8.8 lbs (8 lbs 12 oz)
6ga = 130 oz = 8.1 lbs (8 lbs 2 oz)
7ga = 120 oz = 7.5 lbs (7 lbs 8 oz)
8ga = 110 oz = 6.9 lbs (6 lbs 14 oz)
9ga = 100 oz = 6.3 lbs (6 lbs 4 oz)
10ga = 90 oz = 5.6 lbs (5 lbs 10 oz)
11ga = 80 oz = 5.0 lbs
12ga = 70 oz = 4.4 lbs (4 lbs 6 oz)
13ga = 60 oz = 3.8 lbs (3 lbs 12 oz)
14ga = 50 oz = 3.1 lbs (3 lbs 2 oz)
15ga = 45 oz = 2.8 lbs (2 lbs 13 oz)
16ga = 40 oz = 2.5 lbs (2 lbs 8 oz)
17ga = 36 oz = 2.3 lbs (2 lb 4 oz)
18ga = 32 oz = 2.0 lbs
19ga = 28 oz = 1.8 lbs (1 lb 12 oz)
20ga = 24 oz = 1.5 lbs (1 lb 8 oz)
21ga = 22 oz = 1.4 lbs (1 lb 6 oz)
22ga = 20 oz = 1.3 lbs (1 lb 4 oz)
23ga = 18 oz = 1.1 lbs (1 lb 2 oz)
24ga = 16 oz = 1.0 lb
25ga = 14 oz = 0.9 lb
26ga = 12 oz = 0.8 lb
27ga = 11 oz = 0.7 lb
28ga = 10 oz = 0.6 lb
29ga = 9 oz = 0.6 lb
30ga = 8 oz = 0.5 lb

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Thanks Chris .

Now explain why my 12 ga shotgun is bigger than my 20 ga

The water really gets muddy when you throw in the 410 ga.

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Boy, there’s lots of info out there… Turns out shotgun gauge is fractions of a pound, not an inch…

That 410 is not actually a “gauge”… 0.410 is the bore size.

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I’m not sure that’s accurate, since I’ve seen several conversion tables that showed gauge as different from the closest fractions…

E.g. 1 gauge=0.2813 inches; 4 gauge=0.2344 inches, unless it’s steel plate, then it’s 0.2243 inches.

http://www.engineeringtoolbox.com/gauge-sheet-d_915.html

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Thank you Brian! You pointed out a pretty serious error. What I had posted is not at all accurate… More coming.

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.”.”

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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.

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Darn, this is a pretty weighty subject.

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Makes sense though going back to it’s beginnings.
Could calculate rail, ship, and wagon load outs easy.
S.U.

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