Sheet Metal Gauge Chart: Steel, Galvanized, Stainless, Aluminum
mm and inch thickness for every gauge number, why aluminum is not steel, and how to stop ordering the wrong sheet.
1. What gauge actually means
Gauge is a thickness number inherited from the old wire-drawing tradition: it counted how many passes a wire took through a draw plate. For sheet metal the rule is the opposite of what intuition suggests — a HIGHER gauge means a THINNER sheet. In the US, the Manufacturers' Standard Gauge (MSG) system is defined by weight, not by direct measurement: 41.82 lb per square foot for every inch of thickness, so steel gauge numbers trace back to a weight-based scale.
2. The chart: gauge to mm by material
Thickness in millimeters by material — same gauge number, four different sheets:
| Gauge | Steel | Galvanized | Stainless | Aluminum |
|---|---|---|---|---|
| 10 | 3.42 | 3.51 | 3.57 | 2.59 |
| 12 | 2.66 | 2.75 | 2.78 | 2.05 |
| 14 | 1.90 | 1.99 | 1.98 | 1.63 |
| 16 | 1.52 | 1.61 | 1.59 | 1.29 |
| 18 | 1.21 | 1.31 | 1.27 | 1.02 |
| 20 | 0.91 | 1.01 | 0.95 | 0.81 |
| 22 | 0.76 | 0.85 | 0.79 | 0.64 |
| 24 | 0.61 | 0.70 | 0.64 | 0.51 |
3. The costly mistake: gauge is not a material
Gauge alone does not define a thickness — it only means something together with the material. 16 gauge steel is 1.52 mm, but 16 gauge aluminum is only 1.29 mm, about 15% thinner. A classic HVAC story: a shop orders 16 ga sheet for ductwork, receives aluminum instead of steel, and the whole run comes out floppy and out of spec. Always specify material AND thickness in mm — "1.5 mm galvanized steel", never just "16 gauge".
4. Gauge vs AWG vs SWG
Three different gauge systems are often confused. Sheet metal uses the Manufacturers' Standard Gauge; AWG (American Wire Gauge) sizes electrical wire; SWG (Imperial Standard Wire Gauge) is the old British standard. A 16 AWG wire and a 16 gauge sheet have nothing in common. ASTM A480, the standard for stainless sheet, explicitly discourages gauge numbers and calls the term archaic — order by decimal inch or mm.
5. A practical workflow
Measure the old sheet with a caliper in mm, order in mm, and convert back to gauge only when an older spec sheet forces you to. When in doubt, write both on the order: gauge for the shop floor, mm for the invoice. The Sheet metal gauge converts both ways for steel, galvanized, stainless and aluminum and adds sheet weight, so quotes and orders always match.
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