# Manufacturers Standard Gauge History

## Gauge Numbers, Thickness & Weight a History

Sheet steel or sheet metal differs from “plate” in thickness with sheet being less than .125 inches thick. Sheet metal also has a much better surface finish than plate. Sheet steel or sheet metal is usually purchased by thickness. When the first rolling mills were built in the 18th century the various manufacturers had to decide what series of thicknesses to offer. Some used a series of diameters in wire gauges and others made up their own gauges. The use of many series led to confusion.

In 1877 the American Institute of Mining Engineers recommended a Standard Decimal Gauge. The gauge number was the thickness of the sheet in thousands of an inch. The series ran from 2 to 22 in increments of 2’s (2, 4, 6, 8, 10, 12, 14, 16, 28, 20, 22) then it ran from 25, 28, 32, 36, 40 from there it moved by 5’s to 100, between 110 and 180 by 15’s and lastly 200, 220, 240, 250.

As an act of Congress in 1893 the U.S. Standard Gauge for sheet iron and steel was established for the purpose of levying taxes and duties. This standard was not based on thickness but mass. The mass of a cubic foot of wrought iron was taken to be 480 pounds avoirdupois (16 ounces to a pound). A 1 foot by 1 foot by 1 inch thick will then weight 40 pounds, or 640 ounces, so a sheet 1/64 inch thick should weigh 10 ounces per square foot.

The gauge numbers started at 7/0, which was set at 320 ounces per square foot (and thus 320/640 = ½ inch thick). From 7/0 to number 0 the gauges differed by 20 ounces (so in thickness by approximately 20/640ths); from #0 to #14, the spread was 10 ounces, from #14 to #16 the spread was 5 ounces; from #16 to #20 four ounces; from #20 to #26 two ounces; #26 to #31 one ounce; #31 to #36 half an ounce; and #36 to #38 a quarter of an ounce.

The system did not work because it was based on wrought iron and not rolled steel, and it was determined the weight of rolled steel was nearer to 501.84 pounds per cubic foot. In 1895 the ASME and the American Railway Master Mechanics Association jointly called for the use of the decimal gauge and the abandonment of other gauges in use at the time.

However, gauge numbers would not die. The steel manufacturers redefined the U.S. Standard gauge numbers interpreted by weight and not by thickness. This is called the “Manufacturer’s Standard Gauge” (MSG) and is the one commonly used for uncoated sheet steel. This is also why 16 gauge which was originally 1/16 inch (.0625”) in the original U.S. Standard gauge, is actually 480/502 in thickness, or .0598 inches.

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