In the metric system, the mass per unit area of all types of paper and paperboard is expressed in terms of grams per square meter (g/m²). This quantity is commonly called grammage in both English and French (ISO 536), though printers in most English-speaking countries still refer to the "weight" of paper.
Typical office paper has 80 g/m², therefore a typical A4 sheet (1⁄16 m²) weighs 5 g.
The unofficial unit symbol "gsm" instead of the standard "g/m²" is also widely encountered in English speaking countries.
Typically grammage is measured in paper mill on-line by Quality Control System (QCS) and verified by laboratory measurement.
Read more about this topic: Paper Density