Paper weight is expressed as the number of pounds for one ream (500 sheets) of paper in that paper's basic sheet size (or basis size). Verifying basis weight based on paper thickness, or caliper, is problematic, because different grades of paper have different weights but often the same thickness. 20 pound bond paper and 50 pound book paper are almost the same thickness, for example. To accurately determine basis weight, use a basis weight paper scale.