What Sheet Metal Gauge means
Gauge is a pre-metric thickness convention that survives in drawings and in purchasing because it is short to say. It is also the single most error-prone unit in sheet metal, for two reasons: the number runs the wrong way (higher gauge, thinner sheet), and the same gauge number maps to different thicknesses for different metals.
Why it matters on the shop floor
The standard for carbon steel — Manufacturers’ Standard Gauge — gives 16 gauge as 1.519 mm. The Brown & Sharpe standard used for aluminium gives 16 gauge as 1.290 mm. A drawing that says "16 GA" without naming the material is therefore ambiguous by roughly 0.23 mm on a 1.5 mm sheet, which is more than the tolerance on many formed parts.
In practice
The fix is routine: dimension sheet thickness in millimetres and mention gauge only as a cross-reference. Where a legacy gauge number has to be honoured, state the standard and the metal alongside it. A conversion chart keeps the working figure unambiguous.
What to remember
- Higher gauge number = thinner sheet; the convention runs opposite to intuition.
- Carbon steel (MSG) and aluminium (B&S) gauge scales disagree — always name the metal.
- Dimension in millimetres on new drawings; keep gauge as a parenthetical cross-reference only.