There are two systems in common use to measure the weight of art paper. The weight in lbs, used commonly in the USA, is the weight of 500 sheets of the "basis size" of a particular paper. Note that the basis size (the size of a basic sheet of paper) is not necessarily the same for different types of paper which makes comparison a little tricky. Grams per square metre (often indicated as gsm or g/m2 ) is used in Europe and is the weight of a square metre sheet of paper in grams. This is illustrated below.
Typical basis sizes are indicated in the table below ..