Wausau has a table for conversion of basis sizes . It gets worse. The 67 lb bristol is thus 147 gsm, and is the weight equivalent of a 54 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. Table showing equivalent weight in grams per square meter for various American paper weights and basis sizes. 241.4 gsm 199.1 gsm 163.0 gsm Bond Paper paper manufactured generally for stationery, letterheads or forms. Distinguishable in the more popular grades by a watermark and excellent writing surface. Brightness the measurement of a paper's light-reflective qualities that affect contrast and halftone reproduction.