Quick Reference: Thickness Equivalents
Mils, Inches, Millimeters, and Sheet Steel Gauge
(See table below.)
In the United States, the standard (thickness) gauge for sheet steel is established by federal law. 15 U.S. Code § 206 states, "For the purpose of securing uniformity the following is established as the only standard gauge for sheet and plate iron and steel in the United States of America..." and goes on to provide a complete table of all the different gauge numbers with their thickness and weight equivalents. When a manufacturer sells steel deck or steel roof panels in the US, their products must comply with these gauge standards. Roof and building specifications often use "gauge" when stating the required material thickness. A structural engineer will call for 20 gauge steel deck, for example, or a roof consultant will specify 24 gauge steel panels to be used in a standing seam metal roof. (Interestingly, aluminum roofing panels do not use gauge, and are described in inch decimals, 0.032" being a common thickness.)
Most other countries simply use millimeters.
If you need to check the thickness of the existing steel roof deck on a building, or maybe you're a homeowner and you'd like to verify the thickness of the metal panels the contractor is planning to use...before the roofers start installing them on your roof...they do make a tool for that.* For an existing roof deck, roof access hatches or HVAC penetrations are good places to find an edge where you can use the tool to measure the deck metal. You should find a factory edge, though. If the sheet steel has been cut through by a portable power saw after it was installed, the edges around the hole will almost always be out of gauge.
The table below is a partial sheet steel gauge table, covering the gauges typically used in building construction.