You'd think that calculating the weight of a particular sheet or number of sheets of paper would be fairly simple; after all, a sheet of paper weighs a certain amount, and X times that amount should equal X times the number of sheets. However, the American system of direct measure known as "basis weight" puts an entirely new and somewhat confusing spin on this seemingly simple procedure. But calculating by basis weight doesn't have to be migraineinducing  you just have to think of it in terms of scaling down from a known quantity.

Research the paper's basis height, width, weight and number of sheets. You can think of the basis as a giant stack of uncut paper sitting in the paper mill. For example, say your paper lists as having a basis height of 20 inches, width of 40 inches, weight of 800 pounds and sheet count of 500. This means that, as measured at the paper mill, a stack of 500 sheets of your paper cut to 20by40 inches weighs exactly 800 pounds.

Determine the poundspersquareinch mass of your basis weight stack, meaning the total area of all the sheets combined. Multiply the height by the width (for our example, 20 x 40 = 800 square inches per sheet), then multiply by the number of sheets in the basis stack (400,000 square inches for all of the sheets combined). Then, divide the basis weight by this number to determine how much one square inch weighs (800 divided by 400,000 equals 0.002 pounds per square inch of paper for this example).

Work backward from the known dimensions of the paper in your intended ream. Measure the size of the paper that you intend to use (say a standard 8.5by11 inch sheet), and multiply those figures together to get a square inch per sheet (93.5 in this case). Multiply that by the pounds per square inch (93.5 x 0.002 = 0.187 pounds) to get the weight of one sheet of paper.

Divide the weight of a ream (20 pounds for this example) by the weight of one sheet of paper (0.187 pound) to get the total number of sheets in that ream (107 in this case). Or, you can multiply the number of known sheets in that ream by the weight per sheet to get a ream weight in pounds. So, for our example: given a basis weight of 800 pounds, a basis height and width of 20 and 40 inches and a basis count of 500 sheets, a 20pound ream of 8.5by11 inch paper contains 107 sheets.
Tips & Warnings
 Manufacturers often ship by what is known as "MWeight." This refers to the weight of 1,000 sheets of paper cut to final dimensions (8.5by11 for the above example) given the basis dimensions and weight.
References
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