Bulk
Also called paper bulk, specific volume
A paper’s thickness set against its weight: its thickness in microns divided by its weight in gsm, expressed in cm³/g. An 80 gsm paper with a bulk of 1.8 is 144 microns thick; at the same weight, the higher the bulk, the thicker the paper.
In practice
Ask for the bulk along with the weight: it is what makes the thickness of the spine, and the same paper is made in several bulks. Munken Print Cream at 80 gsm, for instance, comes in bulk 1.5, 1.8 and 2, that is 120, 144 and 160 microns a sheet. A high-bulk paper commonly has a bulk of 1.5 to 2, an offset paper 1.2 to 1.3, and a coated paper stays near 1, even one sold for its bulk. Have the printer work out the spine, from the bulk of the paper chosen, before the cover is finalised.
