A gauge number is not a measurement — it is an index into a lookup table, and the table differs by metal. This reference converts gauge numbers into real thickness in inches, millimetres, and mils for the five most common sheet materials.
How it works
Two main gauge systems are in use. Steel and stainless follow the Manufacturers’ Standard Gauge, where galvanised steel adds a thin zinc-coating allowance on top of the bare-steel value. Aluminium, copper, and brass follow the Brown & Sharpe (B&S, also called AWG) gauge, which produces entirely different thicknesses for the same number.
The scale is counter-intuitive in two ways: the number runs backwards (higher gauge = thinner sheet), and the steps are roughly geometric rather than linear. Once the thickness in inches is known, the metric value is a simple unit conversion:
mm = inch × 25.4
mils = inch × 1000The tool stores the published nominal inch thickness for each gauge and material and derives the mm and mils columns from it.
Example comparison at gauge 16
A common fabrication question is “how thick is 16 gauge?” — and the honest answer is “of what?” 16 gauge standard steel is 0.0598 in (1.52 mm), 16 gauge galvanised steel is 0.0635 in (1.61 mm), but 16 gauge aluminium is only 0.0508 in (1.29 mm). Always state the material with the gauge.
Common gauges and their practical uses
Different gauge ranges have drifted toward particular applications over decades of industry practice:
| Gauge range | Steel use | Aluminium use |
|---|---|---|
| 26–28 | HVAC duct, roofing panels | Thin decorative trim |
| 22–24 | Automotive body panels, enclosures | Aircraft skin, cookware |
| 18–20 | Structural brackets, industrial enclosures | Structural sheet, marine |
| 14–16 | Heavy structural, frames | Heavy tooling plates |
| 10–12 | Very heavy plate, armour substitute | Heavy transport |
These are general patterns rather than rules. Always verify the specification for your application, since standards bodies and industries sometimes differ in what they call out.
Ordering and tolerance notes
Real sheet carries a thickness tolerance — the published gauge thickness is a nominal value, and actual material may run slightly above or below it. For precision parts, specify a minimum and maximum thickness in inches or millimetres on the drawing rather than relying on the gauge number alone. When quoting from a supplier, confirm which gauge system they use: a steel supplier and an aluminium supplier may interpret the same gauge number against different tables, leading to material that is measurably wrong for your design.
When a drawing specifies gauge without naming the material or system, clarify before ordering tooling. Cutting punches or dies to the wrong thickness specification is an expensive mistake.
Mils explained
A mil is one thousandth of an inch (not a millimetre). It appears in coating specifications, PCB trace thickness, and film thickness standards. The table includes the mils column so you can directly compare a sheet’s bare thickness with a coating specification that is also expressed in mils.