Generally single and double-sided PCB board copper foil (copper cladding) thickness of about 35um (1.4mil). There is also another specification of 50um is not common. Multilayer board surface layer is generally 35um (1.4mil), the inner layer of 17.5um (0.7mil).

The thickness of copper foil is also expressed in OZ (ounces), 1OZ means 1OZ of copper evenly covering an area of 1 square foot of copper thickness, which is about 1.4 mil. that's why people always find those sizes odd.

Complete answer! Hope this helps!