What thickness is that gage?

If you find yourself caught between tradition and current industry practice for specifying sheet metal thicknesses you are not alone. It has been years since industry standards have abandoned gage to designate metal thickness, now the standards rely on decimal inch thickness instead.

Some standards attempt to help with the conversion. These standards use exact decimal thickness as the standard, permit rounded mil thickness, and then show the equivalent gage. Gage thicknesses are included for reference only, not as the standard. No wonder confusion reigns.

For an explanation of the derivation of gage thicknesses see "Where do gages come from?"

Some manufacturers continue to publish literature with gage thickness, only, despite the industry standards. Architects continue to specify sheet metal by gage thickness. Familiar terms and old habits are hard to overcome.

Do you rely on one of the many tables found on the internet or Architectural Graphic Standards when you need to convert metal thickness from gage to decimal thickness or vice versa? If you do, the results may be inconsistent with industry standards for specific products. There are multiple gage standards, and different standards for different metals.

Sheet Metal - General
The standards used to specify sheet metal, as a material, do not specify thicknesses. The standards specify physical material properties, leaving the thickness to the purchaser and the manufacturer.

• Steel: ASTM A1008 for cold rolled sheet, and ASTM A1011 for hot rolled sheet
• Galvanized Steel: ASTM A653
• Stainless Steel: ASTM A240 and ASTM A666
• Aluminum: ASTM B209

All of these standards require the sheet metal to be ordered by thicknesses not by gage. Each standard sets specific tolerances for the thickness using decimal inch and millimeter measurements. These material standards apply to most, if not all, products fabricated from sheet metal.

Hollow Metal Doors and Frames
Hollow Metal Manufacturers Association (HMMA) and Steel Door Institute (SDI) publish the standards governing hollow metal doors and frames.

HMMA 803 provides a table for converting sheet metal gage to decimal inch and millimeter thicknesses. This standard states that the "minimum thickness table is to be used instead of discontinued gage numbers."

ANSI/SDI 250.8 for standard steel doors and frames includes the following chart for face sheet with the caveat that the gage thickness is given for reference only.

Door frame thickness for both HMMA and SDI the door fame thicknesses are one thickness greater, except for SDI Level 3 and Level 4 doors. The sheet metal thickness for Level 3 and Level 4 door frames is the same as the door face sheet for the same Level.

Metal Studs
Metal studs are governed by two different standards that permit two different, but overlapping, metal thickness ranges. These standards are explained in an archived Tech Tips - Metal Framing Thickness or Gage.  While the metal door industry uses thicknesses measured in thousandths of an inch, the metal stud framing industry uses measurements to the ten-thousandths of an inch - making the measurements not necessarily interchangeable.

For our own sanity, I would prefer the industry use the rounded mil thicknesses rather than trying to remember four significant digits.

Sheet Metal Flashing

If you know how SMACNA defines sheet metal thickness in the current standard, please add a comment.

Industry Conversion
Complete abandonment of gage thickness by the US construction industry will be a long time coming - perhaps at the same time metric measurement if fully implemented. Meanwhile, be sure to use the standards adopted by the specific industry for which the sheet metal is specified to minimize confusion.