The thickness of a single sheet of paper, measured in thousandths of an inch or in points, where one point equals one thousandth of an inch. In the metric system, caliper is measured in millimeters or micrometers. Caliper is measured using a micrometer, a device which measures minute thicknesses such as paper by applying a static load for a set period of time. As paper is compressible, careful measurements are required, frequently at different points on the paper.
Consistency of caliper throughout the paper web is an important consideration. An abrupt increase or decrease in caliper can affect the extent to which the printing plate or blanket contacts the paper and transfers a complete printed image, as well as other printability and runnability issues. Variations in caliper within a single paper roll cause problems in feeding web offset presses and can cause web breaks.
The thickness of a paper itself varies according to the basis weight desired and other end-use considerations. Caliper can be reduced at a variety of stages in the papermaking process, such as by reducing the amount of furnish deposited on the forming wire, and by increasing the degree of wet pressing, calendering, and supercalendering. Caliper is related to other paper properties, and an increase or decrease in thickness affects other properties, sometimes to their detriment. (See Paper and Papermaking: Paper Properties.)