PCB copper thickness

In US, thickness of a copper layer is expressed in oz/ft². This indicates the amount of copper in oz (unit of weight) to cover an area of 1 ft².

Remembering that $$1oz=28.3495g \quad and \quad 1 ft^2=929.0304cm^2$$ $$1oz/ft^2=0.03051514783g/cm^2$$ and knowing that $$\rho_{Cu} = 8.94 g/cm^3$$ we can calculate the value in um of 1oz/ft² $$1oz_{Cu} \rightarrow \frac{28.3495g}{8.94 g/cm^3}=3.171cm^3 \rightarrow \frac{3.171cm^3}{929.0304cm^2}=34\mu m$$