It is the weight of 500 sheets of paper cut to a basis size. So, 500 sheets of 25" x 38", 60# offset weighs 60 lbs. The basis size for bond is 17" x 22"; for text, offset, and coated 25" x 38"; and for cover 20" x 26". That’s why two similar sheets of different grades may have very different basis weights, for instance, 24# bond and 50# offset. Basis weight is the total weight, in pounds, of a ream of paper. A ream of paper is 500 sheets and is the standard ordering unit for paper. Because paper comes in different sizes and widths, the basis weight of paper can vary from ream to ream. One way to calculate basis weight involves using both metric and customary (inches) units in a formula. The weight will obviously depend on the type of paper you are printing on, assuming 20# text weight, than 2.5 ounces would be correct. However, postage on a 9x12 envelope is significantly different than a business size #10 envelope and I doubt you are folding and squeezing a 12 page document into a #10 envelope. With 5 sheets of paper, it nudges just over 1 ounce (1 to 1.05 ounces), so by the time you add the weight of the stamp(s) and any printing (ink or toner), you'd definitely be over 1 ounce. Also, the 2 ounce postage rate (within the USA) is NOT equal to 2 stamps.