Verified by Rachel Mayfield, Supply Chain Analyst - August 2026
Sheet metal gauge numbers are a trap: the same gauge number means a DIFFERENT thickness depending on the material. 18 gauge is 1.21 mm in mild steel, 1.27 mm in stainless and 1.02 mm in aluminium. The table below gives all three scales side by side, in inches and millimetres, so the material column you read matches the material you are buying. Higher gauge number always means thinner sheet.
Sheet metal gauge to thickness (inches / mm)
| Gauge | Mild steel (in / mm) | Stainless steel (in / mm) | Aluminium (in / mm) |
|---|
| 7 | 0.1793" / 4.55 | - | - |
| 8 | 0.1644" / 4.18 | 0.1719" / 4.37 | 0.1285" / 3.26 |
| 9 | 0.1495" / 3.80 | 0.1563" / 3.97 | 0.1144" / 2.91 |
| 10 | 0.1345" / 3.42 | 0.1406" / 3.57 | 0.1019" / 2.59 |
| 11 | 0.1196" / 3.04 | 0.1250" / 3.17 | 0.0907" / 2.30 |
| 12 | 0.1046" / 2.66 | 0.1094" / 2.78 | 0.0808" / 2.05 |
| 14 | 0.0747" / 1.90 | 0.0781" / 1.98 | 0.0641" / 1.63 |
| 16 | 0.0598" / 1.52 | 0.0625" / 1.59 | 0.0508" / 1.29 |
| 18 | 0.0478" / 1.21 | 0.0500" / 1.27 | 0.0403" / 1.02 |
| 20 | 0.0359" / 0.91 | 0.0375" / 0.95 | 0.0320" / 0.81 |
| 22 | 0.0299" / 0.76 | 0.0313" / 0.80 | 0.0253" / 0.64 |
| 24 | 0.0239" / 0.61 | 0.0250" / 0.64 | 0.0201" / 0.51 |
| 26 | 0.0179" / 0.45 | 0.0188" / 0.48 | 0.0159" / 0.40 |
| 28 | 0.0149" / 0.38 | 0.0156" / 0.40 | 0.0126" / 0.32 |
| 30 | 0.0120" / 0.30 | 0.0125" / 0.32 | 0.0100" / 0.25 |
Why the three scales differ
The steel column is the US Manufacturers' Standard Gauge, historically derived from weight per square foot at steel's density. Stainless uses its own slightly heavier scale, and aluminium follows a scale derived from the Brown and Sharpe (AWG) wire system. None of them are defined in millimetres, which is why converted values look arbitrary. UK and EU suppliers increasingly quote sheet in straight millimetres - if a drawing mixes gauge and mm, confirm which scale the gauge refers to before cutting.
Buying advice
Specify sheet in millimetres wherever you can and treat gauge as a legacy label. If a supplier lists "1.2 mm (18g)" for steel that is consistent; "1.2 mm (18g)" for aluminium is not (18g aluminium is 1.02 mm) and is worth a query. Tolerances also matter: commercial sheet tolerance can be 10% of nominal thickness, so measure critical stock with a micrometer on arrival.