Why is Sheet Metal Measured by Gauge Instead of Inches?

Have you ever wondered why carpenters get to work with 1/2″ drywall but metal fabricators have to specify sheet metal thickness in terms of gauge? It’s a legitimate question that a lot of folks who are new to metal fabrication wrestle with. To make matters more confusing, the range of gauge sizes works opposite of conventional logic, with larger numerical values representing thinner material and vice versa. And if that weren’t enough, the actual measured thickness for a given gauge of sheet metal varies depending on the type of material it is.

So how did we get here and why are we still using this system?

The “how” part of the question starts at the height of the American Industrial Revolution. In 1893, the U.S. Code was amended to include Title 15 Chapter 6 Sub-Chapter III Section 206, which established a “standard gauge for sheet and plate iron and steel” to standardize the weight of materials weight for the purposes of tax collection. Prior to this, different manufacturers used their own measuring systems to sell their materials, leading to inconsistencies in the market. This system, known as the U.S. Standard Gauge, was very similar to the American Wire Gauge (AWG) developed by Brown & Sharpe in 1857, but specific to the nature of sheet metal instead of single-strand round wire.

The code outlined a 45-step chart of material thicknesses from 1/2” (0000000 or “7 aught”) to 1/160” (38-gauge) for these. The different gauges on the chart represented (at least on some level) the number of reduction steps, or draws, required to squeeze the material into ever thinner sheets. Think of it like the number of passes a rolling pin makes to work a lump of dough into a thin pie crust. That’s why 20-gauge steel is thinner than, say, 10-gauge.

As a result, for each “gauge” there is a consistent weight for a given square inch or square foot of a given material, regardless of whose mill produces it. That’s also why different materials have their own gauge specs. If you measure a piece of 14-gauge carbon steel with a micrometer, it will be 0.0747” thick, while a piece of 14-gauge stainless steel will measure out to 0.0751”. That’s because carbon steel has greater density than stainless. Likewise sheet aluminum, brass, and copper all have their own gauge charts as well.

So in this age of laser measurements and digital micrometers, why do we still use the U.S. Standard Gauge at all? The answer largely comes down to familiarity. After 130 years of standardization, it’s what the industry knows and has built its tooling for. Nevertheless, some countries use nominal measurements (typically in metric figures) for both sheet metal and wire, while others have adopted their own standards over time.

Since the system apparently isn’t broken, we don’t expect it to change anytime soon. What’s important is that you understand how to read to a sheet metal gauge and that you know what material you’re working with.