18 gage steel is 0.0478 inches thick 18 gage steel is 0.0478 inches thick
The thickness of 16 gauge steel in US standards is .0625 inches or 1.59 mm and according to UK standards is .065 inches or 1.651 mm.
The actual thickness associated with a "gauge range" varies largely by industry. Gauges are OK for ball-parking, but are not adequate for engineering purposes. Producing mills and reputable service...
0375 inches or .953 mm
A 21 Gage Steel plate is 1/4 inch thick.