from Stephen Brown <firstname.lastname@example.org>: The American measures of paper weights are nothing if not confusing. The weight (e.g. 80 pounds) of a paper refers to the weight of 500 sheets in the paper's basis size. The basis size varies with the type of paper. To convert to the more rational metric measure, you have to know both the weight and the basis size. Thus, for cover stock, the basis size is 20" by 26". A ream (500 sheets) is thus 500 x 520 square inches, or a total area of 260,000 square inches. Eighty pound cover stock is 80 lb per 260,000 square inches, or about 308 micropounds/square inch. One lb/square inch is about 704 kg/square meter; thus 80 pound cover stock is 216.5 gsm.
Twenty four pound bond or writing paper is measured with a basis size of 17" by 22", or 187,000 square inches per ream. Since the basis size is smaller than for cover stock, a sheet of bond paper would be heavier than a sheet of cover stock of the same nominal weight. 24 lb bond is about 90 gsm; 24 lb cover would be about 65 gsm. But cover stock isn't sold in a 24 lb weight, so in practice one rarely makes this sort of comparison. Wausau has a table for conversion of basis sizes.
It gets worse. The Wausau paper I've mentioned is sold as cover paper, but actually measured on a bristol basis, 22.5" by 28.5", or about 320,625 square inches per ream. The 67 lb bristol is thus 147 gsm, and is the weight equivalent of a 54 lb cover stock. It has a caliper (thickness in mils) of 8.4, approximately the same as Wilhelmshaven and JSC models, and very close to stuff I've seen identified as "65 lb cover stock". I'm beginning to suspect a certain sloppiness in distinguishing between cover stock and bristol stock in this country.
Caliper, of course, is an imperfect guide. Paper varies in density, so the same thickness can have differing weights. It has the virtue, however, of being easy to measure and compare.
|Basis size (inches)|
|20||75.3 gsm||54.1 gsm||43.9 gsm||36.2 gsm||29.6 gsm|
|24||90.3 gsm||65.0 gsm||52.7 gsm||43.4 gsm||35.6 gsm|
|28||105.4 gsm||75.8 gsm||61.5 gsm||50.7 gsm||41.5 gsm|
|60||225.8 gsm||162.4 gsm||131.7 gsm||108.6 gsm||88.9 gsm|
|67||252.1 gsm||181.3 gsm||147.1 gsm||121.2 gsm||99.3 gsm|
|80||301.0 gsm||216.5 gsm||175.6 gsm||144.8 gsm||118.5 gsm|
|110||413.9 gsm||297.7 gsm||241.4 gsm||199.1 gsm||163.0 gsm|
email@example.com| Steve Brown |