The area density (also known as areal density, surface density, superficial density, column density, or density thickness) of a two-dimensional object is defined as the quotient of mass by area. The SI derived unit is the "kilogram per square metre" (unit symbol kg·m⁻²).

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").

A generalized areic quantity is defined as the quotient of a generic physical quantity by area, such as surface charge density or areic electric charge. A related area number density can be defined by replacing mass by number of particles or other countable quantity.

Area density can be calculated as: $${\displaystyle \rho _{A}={\frac {m}{A}}}$$ or $${\displaystyle \rho _{A}=\rho \cdot l,}$$ where ρ_A is the average area density, m is the total mass of the object, A is the total area of the object, ρ is the average density, and l is the average thickness of the object.

A special type of area density is called column density (also columnar mass density or simply column density), denoted ρ_A or σ. It is the mass of substance per unit area integrated along a path; It is obtained integrating volumetric density ${\displaystyle \rho }$ over a column: $${\displaystyle \sigma =\int \rho \,\mathrm {d} s.}$$

In general the integration path can be slant or oblique incidence (as in, for example, line of sight propagation in atmospheric physics). A common special case is a vertical path, from the bottom to the top of the medium: $${\displaystyle \sigma =\int \rho \,\mathrm {d} z,}$$ where ${\displaystyle z}$ denotes the vertical coordinate (e.g., height or depth).

Columnar density ${\displaystyle \rho _{A}}$ is closely related to the vertically averaged volumetric density ${\displaystyle {\bar {\rho }}}$ as $${\displaystyle {\bar {\rho }}={\frac {\rho _{A}}{\Delta z}},}$$ where ${\textstyle \Delta z=\int 1\,\mathrm {d} z}$; ${\displaystyle {\bar {\rho }}}$, ${\displaystyle \rho _{A}}$, and ${\displaystyle \Delta z}$ have units of, for example, grams per cubic metre, grams per square metre, and metres, respectively.

It is a quantity commonly retrieved by remote sensing instruments, for instance the Total Ozone Mapping Spectrometer (TOMS) which retrieves ozone columns around the globe. Columns are also returned by the differential optical absorption spectroscopy (DOAS) method and are a common retrieval product from nadir-looking microwave radiometers.