The SI derived unit is the "kilogram per square metre" (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 related area number density can be defined by replacing mass by number of particles or other countable quantity, with resulting units of m−2.

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:

is closely related to the vertically averaged volumetric density

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[3] and are a common retrieval product from nadir-looking microwave radiometers.

[4][5] A closely related concept is that of ice or liquid water path, which specifies the volume per unit area or depth instead of mass per unit area, thus the two are related:

In astronomy, the column density is generally used to indicate the number of atoms or molecules per square cm (cm2) along the line of sight in a particular direction, as derived from observations of e.g. the 21-cm hydrogen line or from observations of a certain molecular species.

[6] The concept of area density can be useful when analysing accretion disks.

In the case of a disk seen face-on, area density for a given area of the disk is defined as column density: that is, either as the mass of substance per unit area integrated along the vertical path that goes through the disk (line-of-sight), from the bottom to the top of the medium:

denotes the vertical coordinate (e.g., height or depth), or as the number or count of a substance—rather than the mass—per unit area integrated along a path (column number density):

The current unit of measure is typically gigabits per square inch.

[7] The area density is often used to describe the thickness of paper; e.g., 80 g/m2 is very common.

It is also sometimes specified in ounces per yard in a standard width for the particular cloth.

When studying bodies falling through air, area density is important because resistance depends on area, and gravitational force is dependent on mass.

The total electron content in the ionosphere is a quantity of type columnar number density.

Snow water equivalent is a quantity of type columnar mass density.