What’s the difference between paper being labelled with lb, #, gsm, g/m2, and g/m2?
Besides a generic “Text” weight or “Cover” weight, paper descriptions use a number to refer to the weight of the paper. The higher the number, the heavier the paper. Heavier paper is typically thicker as well.
There are two systems for indicating the weight of paper; an international metric system (ISO) and the North American ANSI and ARCH system. The North American system for paper weight uses pounds (expressed as either # or lb).
The North American pound rating, also called a basis weight, is based on the weight of 500 sheets (a.k.a. a ream) of a specific size.
There are 5 basis weight categories:
- Text (Basis size = 25 x 38”)
- Index (Basis size = 25.5 x 30.5”)
- Bristol (Basis size = 22.5 x 28.5”)
- Cover (Basis size = 20 x 26”)
- Bond (Basis size = 17 x 22”)
The U.S. system is a bit confusing because the same pound number (weight) can be used for both lighter (Text) paper and heavier (Cover) paper. For example, 80# Text paper and 80# Cover paper have the same pound number even though the Cover stock is almost twice as heavy!
The reason for this is the text stock “basis” uses 500 sheets measuring 25″ x 38. The cover stock “basis” uses 500 sheets measuring 20″ x 26″.
The metric ISO system in comparison is more straight forward because it uses one “basis” unit of reference. The metric system uses grams per square meter (gsm or g/m2 or g/m2), often called “grammage”. The ISO rating is based on the weight of a single sheet of paper that measures 1×1 meter. Simple.
For example, 80# Text Paper weighs 104 g/m2 while 80# Cover Stock weighs 218 g/m2. The Cover stock is clearly more than twice as heavy as the Text stock.