Sheet Metal Gauge and Thickness Conversion

In the United States and Canada, steel roofing is usually described by gauge, such as 22, 24, 26 or 29 gauge. The gauge number is a traditional, non-linear scale rooted in weight per area, and a larger number means a thinner sheet. Most of the world specifies sheet directly in millimetres, and standards bodies including ASTM discourage gauge numbers, calling the term archaic. Copper is a further exception in North America: it is sold by weight in ounces per square foot.

Gauge does not mean the same thing for every metal

The same gauge number gives different thicknesses for plain steel, galvanized steel and aluminium. Typical published values:

Gauge	Steel sheet	Galvanized steel	Aluminium
22	0.0299 in (0.76 mm)	0.0336 in (0.85 mm)	0.025 in (0.64 mm)
24	0.0239 in (0.61 mm)	0.0276 in (0.70 mm)	0.020 in (0.51 mm)
26	0.0179 in (0.45 mm)	0.0217 in (0.55 mm)	0.017 in (0.43 mm)
28	0.0149 in (0.38 mm)	0.0187 in (0.47 mm)	0.0126 in (0.32 mm)
29	0.0135 in (0.34 mm)	0.0172 in (0.44 mm)	0.0113 in (0.29 mm)

Published figures for coated roofing sheet can differ slightly from these tables, partly because thickness may be stated with or without the metallic coating, as the plain and galvanized columns show. Always check the actual specified thickness.

Copper weights

• 16 oz/ft2 copper: about 0.0216 in (0.55 mm), roughly 24 gauge.
• 20 oz/ft2 copper: about 0.0270 in (0.69 mm), roughly 22 gauge.

Aluminium roofing

Although aluminium gauge tables exist, aluminium roofing is normally quoted in thousandths of an inch or in millimetres, because a steel gauge number has no meaningful aluminium equivalent. Residential aluminium roofing typically runs from about 0.019 in (0.48 mm) for flashing and some stamped shingles to about 0.040 in (1.02 mm), with standing seam pans often 0.032 in (0.81 mm) or heavier. Temper and profile shape also affect stiffness, so two sheets of the same thickness can perform differently.