How thick is 26 gauge steel in mm?

For a wire classified under American Wire Gauge standards, 26 gauge wire is 0.0159" (0.40386 mm) in dameter.

For a wire classified undere metric wire gauge standards, a 26 gauge wire is 2.6mm in diameter. Metric gauges are calculated simply by multiplying the diameter, in mm, by 10 and therefore increase as the diameter increases, unlike the AWG standard.