Area density

The area density (also known as areal density, surface density, superficial density, areic density, mass thickness, column density, or density thickness) of a two-dimensional object is calculated as the mass per unit area. The SI derived unit is the kilogram per square metre (kg·m⁻²). A related area number density can be defined by replacing mass in by number of particles or other countable quantity.

Area density
Common symbols	${\displaystyle \rho _{A}}$
SI unit	kg/m2
In SI base units	m−2⋅kg
Derivations from other quantities	${\displaystyle \rho _{A}={\frac {m}{A}}}$ ${\displaystyle \rho _{A}=\rho l}$
Dimension	${\displaystyle {\mathsf {L}}^{-2}{\mathsf {M}}}$

In the paper and fabric industries, it is called grammage and is expressed in grams per square meter (g/m²); for paper in particular, it may be expressed as pounds per ream of standard sizes ("basis ream").