A term used specifically in book manufacturing which refers to the number of sheets that will occupy one inch of thickness when compressed at 36 pounds per square inch (psi) for thirty seconds. The test to determine bulking number is performed to give an indication of what paper thickness (also called caliper or bulk) is required in order for the bound book to fit in its binding. The bulking test is designed to simulate the pressure imparted to the paper by the bookbinding process (called smashing or casing-in). Measuring the caliper of a single sheet is impractical for book manufacturing, as its smashed bulk, or the bulk that will result when pages are subjected to the high pressure of a binding, will be less than the number obtained by simply multiplying the thickness of one sheet by the total number of sheets in the book, due to a "nesting" or packing effect caused by the pressure of a binding. The bulking number is multiplied by two to provide pages per inch (since each sheet contains two pages, a verso and a recto). (See also Bulk, Caliper, and Bulking Index.)
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