As it always does, the metric system provides an easy and sensible way to measure the thickness of materials such as sheet metal and plastic sheeting.

However, the United States, being one of the world's largest traders in these commodities, if not the largest, is still unofficially committed to Imperial units, so industries around the world must regularly convert between metric thickness units and gauge measurements, which have more or less standard correlations with Imperial units.

The correlation between gauge number and thickness is fairly straightforward in the plastic sheeting market, but when it comes to sheet metal, the correlations vary with the metal in question. If you're looking for material that is 51 microns thick, for example, you'll have an easy time hitching that to a gauge number when you purchase plastic, but you probably won't find sheet metal than thin.

When it comes to metric units, the prefix "micro-" denotes one millionth of the base unit, which is the meter when measuring length, distance or thickness. A micrometer (µm) is simply 10^-6 meters, and to further simplify matters, the term is usually shortened to microns. A plastic or metal sheet with a thickness of 10 microns (µm) is 10^-5 meters thick. End of story.

HVAC technicians also use the term "microns" to refer to pressure, especially when evaluating the low pressure in a vacuum situation. Here the term "micron" refers to the length of a mercury column, and because it's such a small unit, gauges calibrated in microns are extremely sensitive.

When it comes to sheet plastic, the unit of choice is the "mil," which refers to one-thousandth of an inch, and there is a direct correlation between the gauge number and the mil. A gauge number of 10 refers to a thickness of 0.1 mils, 20-gauge refers to 0.2 mils and so on. Consequently, to make the conversion to metric units, you just need to know the number of microns in a mil, which is 25.4.

Consequently, if you're looking for the number of microns in 10 gauge plastic, just multiply the number of mils (0.1) by the micron-to-mils conversion factor (25.4). The answer is 2.54 microns.

Going the other way, if you have sheet plastic that is 51 microns thick, you can find the number of mils, and the gauge number, by dividing by the conversion factor. It turns out to be 0.2 mils, or 2-gauge.

Things aren't so simple when evaluating metal thickness for three reasons: Metal gauge numbers aren't directly correlated to inches, they are different for different metals and they decrease with increasing thickness. Consider galvanized steel, for example. The minimum gauge number is 8, and the maximum is 30, which corresponds to 0.168 and 0.0157 inches respectively, or 4,269 µ and 398 µ respectively.

When it comes to aluminum, the minimum gauge is 7, corresponding to 0.144 inches, or 3,665 µ, and the maximum gauge is 30, which corresponds to 0.01 inches, or 0.255 µ. Brass has the same gauge-to-thickness correlations as aluminum, but copper has different ones. The minimum gauge for copper is 7 (0.18 in; 4,572 µ), and the maximum gauge is 30 (o.012 in; 305 µ).

For any type of sheet metal, 51 microns is extremely thin and falls outside the gauge classification. In general, metals thinner than 300 microns are too thin to use and aren't sold on the market. If you want to know the thickness of a standard gauge of any metal sheeting, your best bet is to look it up in one of the many tables available online.