Basis weight
Basis weight turns up in 2 fields: paper optics, nonwovens. Here is what it means to the people in each one.
Basis weight in paper optics
Weight of a 500-sheet ream cut to a grade's basic sheet size, used in North American paper trade.
Stated in pounds per ream, it depends on the basic size assigned to each grade family: 17 x 22 inches for bond, 25 x 38 inches for book and offset, 20 x 26 inches for cover. That is why 20 lb bond and 50 lb offset weigh about the same per square meter, near 75 g/m2. Mill people often use basis weight loosely to mean grammage, which confuses metric customers.
"It's 50-pound offset in basis weight, which is 74 grams for the guys in the Montreal office."
Basis weight in nonwovens
Mass of a nonwoven per unit area, almost always given in grams per square meter.
Basis weight is measured by cutting and weighing a set area of fabric, and it is the primary spec on nearly every nonwoven. Hygiene spunbond runs as light as 8 to 15 g/m2, wipes about 30 to 80 g/m2, and needlepunched geotextiles and felts from 100 to well over 1,000. Customers usually specify both an average and a limit on variation across and along the roll.
"Basis weight is 1.5 grams heavy on the operator side, so we're giving away polymer on every roll."