21 Gauge Sheet Metal

Steel (MSG) 0.0329″

Galvanized 0.0366″

Stainless 0.0344″

Aluminum 0.0285″

Steel: 0.84 mm · Galvanized: 0.93 mm · Stainless: 0.87 mm · Aluminum: 0.72 mm . The same gauge number is a different thickness in each material — always confirm which scale a drawing means, or better, specify decimal thickness.

Neighboring gauges (steel column)

Gauge	Steel	Galvanized	Stainless	Aluminum
19	0.0418	0.0456	0.0437	0.0359
20	0.0359	0.0396	0.0375	0.0320
21	0.0329	0.0366	0.0344	0.0285
22	0.0299	0.0336	0.0312	0.0253
23	0.0269	0.0306	0.0281	0.0226

Measuring wire, not sheet? Wire uses the AWG scale — see AWG 21 (0.0285″ Ø). Aluminum sheet happens to share the AWG scale, which is why the aluminum column matches it.

Data provenance. Sheet-metal gauge tables: Manufacturers’ Standard Gauge (steel), galvanized, and stainless (Industry reference). ASTM B258, Standard Specification for Nominal Diameters of AWG Sizes (ASTM International). Steel per Manufacturers' Standard Gauge; stainless per the conventional supplier table (no governing standard); aluminum per B&S (= AWG, ASTM B258). Sheets carry rolling tolerances. See all data sources.