The Changing of Standards
Caliper and PPI ratings can get confused as they both are a measure of thickness, but the PPI is the mills’ target for production. Caliper refers to the thickness of a sheet of paper, expressed in a thousandth of an inch. This measurement is taken with a micrometer. Normally, paper caliper should not have more than a plus or minus 5-percent variance within a sheet. Normal paper manufacturing tolerance within a paper production run is plus or minus 5-percent to 7-percent caliper. The PPI is calculated by dividing the number two by the caliper. (Don’t confuse pages per inch with sheets per inch; there are two pages, front and back, per sheet.)
By incorporating groundwood fibers, the paper bulks to a higher PPI at a lighter basis weight. This is a natural phenomenon of groundwood additives, as is the increased opacity. Both characteristics allow a user to buy a paper that weighs 10-percent to 20-percent less, but has the same PPI and opacity as heavier stocks.
The basis weight is the designated fixed weight of 500 sheets, measured in pounds, in that paper’s basic sheet size. It is important to note that the “basic sheet size” is not the same for all types of paper. Generally, the relation between caliper and basis weight is: the greater the caliper (the thicker the paper), the greater the paper weight.
Books are expected to last a long time, and freesheet paper grades meet the American National Standards Institute (ANSI) for permanency; uncoated groundwoods do not.
The ANSI “Standard Guide for Selection of Permanent and Durable Offset and Book Papers” outlines offset and book-paper-permanency specifications for coated and uncoated stocks. These standards are used for books and other permanent records that are expected to last several hundred years in a library with little change in readability or handling properties.