If you look at each gage and the tolerances, you will find that there is some overlap between the gages. The info below is out of the "Machinery's Handbook, 21st edition, page 467" for sheet metal. When you call your metal dealer, ask them if they don't have 19 gage for the thickness of 18 gage. It might measure close enough to the upper tolerance for 19 gage to work for you. Even it is a little more, it might not be that much of a difference. Most of us would rather work with a little extra instead of less. Several us have been talking about a metal purchase on the west coast, you would be surprised on some of the metal thicknesses we have been quoted. The only way that you will know for sure is to mic it and it will vary some. The tolerance for these gages is +/- .004, this was from the table on page 467 for Cold Rolled Alloy Steel (for coils and cut lengths) between 40 and 48 inches wide and between .0314 and .0508 thick.
i believe the gauge stated by vendors is a approximation, most of our sheet metal comes from asia now and are speced in millimeters and not gauges from the manufactures
actually I have a recent example, I just purchased lower inner and outer door replacement panels for my F100, the vendor told me that its considered 18 gauge however since they are made in asia and in millimeters that its the closest they could get to actual 18 gauge, their manufacturing mm thickness increments don't match up with our gauge increments, its definitely something to be aware of
I reason for my post was to show what the variations could be in the gages. Remember a mm = 0.03937 inches and everything has a tolerance. It doesn't matter is something is made in millimeters, it still has an inch equivalent thickness; which would coincide with a gage specification.
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