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: 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 (gsm); for paper in particular, it may be expressed as pounds per ream of standard sizes ("basis ream").