Metal Stud Gauge Chart: Mils, Gauges and Sizes Explained

If you have looked at a metal stud chart, you have probably seen the same steel described three ways: by gauge, by mils and by decimal inches. That is the source of a lot of confusion, and in steel framing that confusion can put the wrong stud in a wall. This guide puts the numbers in one place and explains how to read them.

Metal stud gauge chart (mils to gauge)

A mil is one thousandth of an inch. The mil number is the minimum base-metal thickness, before coating, and it equals 95% of the design thickness engineers use in calculations.

Thickness and color values are from the MarinoWARE gauge card and STRUCTURE magazine's jobsite guide, listed below. Color coding is common practice, but always confirm against the stamped marking on the member.

How to convert metal stud gauge to mils

Find the gauge in the second column and read the mil value on the same row, but check which product it is. For structural studs, 20 gauge is 33 mils, 18 is 43, 16 is 54, 14 is 68, 12 is 97 and 10 is 118. For drywall studs, "20 gauge" is usually 30 mils, and "EQ" studs can be thinner still. To decode a full stamp, try our stud decoder.

Why we specify by mils, not gauge

Gauge is an old system that doesn't map neatly onto modern steel framing. "20 gauge" can mean a 33-mil structural stud, a 30-mil drywall stud, or a much thinner proprietary stud marketed as equivalent. Mils are a direct measurement, and they are what codes, engineering standards and stamped drawings use.

Our rule is simple: specify by mils. If a drawing, quote or order uses gauge only, ask for the mil thickness before anything is installed.

How to read a stud designation

Steel framing members carry a standard designation. Using 600S162-54 as an example:

Common web depths include 250 (2-1/2 in.), 362 (3-5/8 in.), 400 (4 in.), 600 (6 in.), 800 (8 in.) and larger sizes for joists. Common flange widths include 125 (1-1/4 in.), 162 (1-5/8 in.), 200 (2 in.) and 250 (2-1/2 in.).

Manufacturers stamp members with this kind of information, often adding the steel's yield strength, such as 50 ksi. For an overview of the members themselves, see what light-gauge steel framing is.

Can metal studs be load bearing?

Yes, if they are structural members. Two ASTM standards separate studs that carry building loads from studs that don't.

• ASTM C955 covers cold-formed steel structural framing members, with a base-steel thickness of at least 0.0329 inch (33 mils). These are designed under AISI S240 and S100 and are what you need for load-bearing walls, exterior walls, floors and roofs.
• ASTM C645 covers nonstructural steel framing members for interior construction, such as partitions, soffits and furring. These are designed under AISI S220.

The "20-gauge" and EQ stud trap

This is where mistakes happen. Drywall studs labeled "20 gauge" are typically 30 mils, not the 33 mils of a 20-gauge structural stud. Some proprietary high-strength studs are sold as "EQ" or "equivalent" gauges. One manufacturer's "20 ga EQ" stud is 18 mils, and its "25 ga EQ" is 15 mils.

EQ studs can be a good product for interior partitions. They are not substitutes for structural studs, and they should never end up in a load-bearing or exterior wall. On a jobsite, a pink-marked or unmarked thin stud in a bearing wall is a red flag.

What gauge metal studs for exterior walls?

Exterior walls resist wind pressure and, in most homes, carry floor and roof loads. They need structural C955 members, sized by design.

Under the California Residential Code's prescriptive wall section (R603), stud thicknesses run from 33 to 68 mils. The code calls for steel of at least 33 ksi for 33- and 43-mil members and at least 50 ksi for 54- and 68-mil members. The right choice depends on wall height, stud depth and spacing, wind exposure and loads above. Many homes fall outside the prescriptive limits or use engineered designs anyway, and every load-bearing frame we build is engineered and stamped by a California-licensed engineer.

The practical answer: use what the stamped drawings call for, in mils, with the specified steel strength and coating. Don't substitute a thinner or lower-strength member because it is "the same gauge."

Interior walls and heavy items

Interior non-load-bearing partitions can use nonstructural C645 studs. Interior walls that support floors or roofs are bearing walls and need structural members.

For TVs, cabinets, grab bars and shelving, plan backing before drywall goes up. Steel or wood blocking between studs, or fasteners rated for steel studs and the load, are the reliable options.

Coatings: G40, G60 and G90

Steel framing is protected by a zinc coating applied at the mill. The "G" number is the total coating weight on both sides of the sheet in hundredths of an ounce per square foot: G40 is 0.40 oz/ft², G60 is 0.60 oz/ft² and G90 is 0.90 oz/ft².

ASTM C645 covers interior construction, so lighter-coated drywall studs don't belong in exterior walls. We frame with G90. If your home is near the ocean or has walls that may see more moisture, the project engineer confirms the coating for your site. Whatever coating you choose, the wall still needs good flashing and drainage. A sound coating is part of why a steel frame can't rot and termites can't eat it; see termite-resistant homes in California.

Checking material on the jobsite

Before framing starts, it is worth confirming:

1. The stamped designation matches the drawings (depth, flange, mils).
2. The steel strength (33 or 50 ksi) matches where specified.
3. The coating meets the specification.
4. Color marks agree with the stamp. If members are unmarked, thickness can be measured with a micrometer.

Getting it right on your project

The thickness of a stud is one line on a drawing, but it drives strength, cost and code compliance. If you are converting wood-frame plans to steel, the engineer will set member sizes for every wall. Learn how that works on our plan conversion page, compare materials in steel vs. wood framing, check your address for fire hazard and climate zone, or contact us with questions about your plans.

Why homeowners choose steel framing with APB

Getting the stud thickness, steel strength and coating right is what makes a steel frame last, and it is the engineer's job to specify them. We build to the stamped drawings in mils, check the markings on the jobsite and don't substitute thinner members for a matching gauge. For the bigger picture, read why steel framing pays off in California, see how plan conversion works, or contact us.