Grammage and basis weight, in the pulp and paper industry, are the area density of a paper product, that is, its mass per unit of area. Two ways of expressing grammage are commonly used:
- Expressed in grams (g) per square meter (g/m2), regardless of its thickness (caliper). This is the measure used in most parts of the world.
- Expressed in terms of the mass (expressed as weight) per number of sheets of a specific paper size, known as basis weight. The convention used in the United States and a few other countries using US-standard paper sizes is pounds (lb) per a ream of 500 (or in some cases 1000) sheets of a given (raw, still uncut) basis size. Japanese paper is expressed as the weight in kilograms (kg) per 1,000 sheets.