How to formally calculate the surface density of layered materials such as graphene? Typically density is defined in units of mass per volume. In the case of graphene it is mass per area, i.e. surface density. What would be the correct way to calculate the surface density (mass per unit area) of graphene, for example?

Would it be correct to multiply the density of graphite by the van der Waals gap of graphite?
The C-C length in graphene is l = 0.142nm and the area of a hexagon can be calculated with the formula:

$ A = \frac{3\sqrt{3}}{2} l^2 =  0.0523nm^2$

In each hexagon, there are 2 full carbon atoms(1/3*6) so the surface density of one single layer is:

$ S_d = \frac{2*massCarbon}{A}  =  \frac{2*1.994 × 10^{-26}Kg}{0.0523× 10^{-18}m^2} = 76.26 × 10^{-8} Kg/m^2 = 7.63× 10^{-8} g/cm^2$  

If you are considering 2, 3, etc. layers than the surface density it's twice, three times etc. the surface density of the single layer.

Additional note: The distance between layers is h = 0.335 nm and therefore its density can be calculated as:

$ d = \frac{S_d}{h}  =  \frac{7.63 × 10^{-8} g/cm^2 }{0.335× 10^{-7}cm} = 2.28 g/cm^3  $  

This is very close to the experimental value I have found online says that the density of graphene is $2.267 g/cm^3$