Steel Buyers Basics

Steel Buyers Basics – Gauge, Thickness & Weight

Within the construction market, which represents more than 50% of the flat rolled steel consumption in the United State, there is often confusion about gauges, gauge numbers, thickness and weights.

The most common gauge system used in both Canada and the United States is the Manufacturers Standard Gauge for Steel Sheets (MSG). The MSG was developed having a definite thickness equivalent for each gauge number. In the standard gauge system the density of steel is 489.6 pounds per cubic foot (one foot wide by one foot long by one foot high). This equates to 40.80 pounds per square foot by 1 inch thick.

Many of you are well aware of another calculation which equals 41.82 pounds per square foot by 1 inch thick. Over a period of time the value for sheet steel (weight) was found to be close to 2.5 percent heavier than the 40.80 lbs/ft squared by 1 inch. This was due to the center of the sheet being slightly heavier than the edges (called a crown) and the 2.5 percent was to provide a more accurate calculation. The 41.82 pounds per square foot by one inch thick has been commonly used to express the relationship between thickness and weight for sheet steel.

Past domestic rolling mills and hot dipped galvanizing lines did not have the sophistication which exists at today’s domestic and most foreign steel mills. The older mills were not able to keep consistent thickness control of the base metal. At the same time the early hot dipped galvanizing lines did not have tight tolerance controls over the zinc coating being applied to the steel. The combination of these two issues brought about gauge charts which referenced steel in theoretical nominal weight terms. Theoretical meaning the thickness was based on an average (+/-) based on historical references. Thus 24 gauge galvanized became .0276” while the standard MSG for cold rolled steel 24 gauge is .0239”.

How did we get from thickness to weight? For 24 gauge galvanized steel we take the .0276” X 41.82” = 1.54232 lbs. Where did the 1.156 lbs per square foot number which is referenced on most gauge charts supplied in the industry come from? The only answer I have (at the moment) is that the .0276 was actually rounded down from .0276423 some time back in history. I am looking for historical references to see if I can find a definitive answer to this question. If you have one or can forward me to more information I would appreciate the help. However, the 1.156 pounds per square foot for 24 gauge galvanized is the most commonly used number in construction related charts such as SMACNA (Sheet Metal and Air Conditioning Contractors National Association) guidelines for galvanized duct.

So, where does the confusion come from? When purchasing from domestic steel mills back in the 1970’s, 1980’s and 1990’s it was common to buy steel either on actual scale weight (you paid for what the coil weighed) or on theoretical minimum weight whereby you paid for the “theoretical” calculated weight. The theoretical minimum calculation was based on the thickness ordered (in inches) and used the 40.80 value for pounds per square foot 1” thick (MSG) times the linear footage and the width of the coil in feet. The key here being the customer paid for the footage received not the actual scale weight of the coil.

At the same time organizations such as SMACNA were putting out gauge (thickness) guidelines for their membership which referenced minimum thicknesses and maximum thicknesses. 24 gauge galvanized became .0236 minimum with a .0296 maximum and a “nominal” decimal of .0276.

A mill buyer could then order the minimum to gauge (.0236) knowing the ASTM tolerances at the time (-0+.006) would not put the material outside the SMACNA guidelines. Back in the 1970’s when I started in the business many mills would roll material to the heavier side of the thickness tolerance either due to poor rolling practices or to take advantage of actual weight billing (the thicker the steel the more the weight – more weight = more billing since steel is sold on weight).

In an effort to compete with mills that had better rolling practices mills began offering theoretical minimum weight billing (TMW) so when a company ordered .0236” galvanized they only paid .0236 X 40.80 or .9629 pounds per square foot.

Then someone figured out there was quite a difference between .9629 pounds per square foot and 1.156 pounds per square foot in 24 gauge galvanized steel.