Caliper refers to the thickness of a sheet of paper expressed in a thousandth of an inch or mil. This measurement is taken with a micro meter. Normally, paper caliper should not have more than a plus or minus 5% variance within a sheet. Generally, the greater the caliper (the thicker the paper), the greater the paper weight. Heavily textured art papers may have higher mil thickness than comparable smooth papers because of the raised texture surface.
When selecting paper stock for posters, ink coverage, dimensions and intended use determine optimum paper thickness. If there are large areas of rich solid colors consider 9+ mil. This is a good weight to consider for very wide or long posters. The extra stiffness makes them easier to handle without denting and tearing. The archival papers recommended by Reactive Imaging range between 9 mil and 16 mil with canvas or highly textured art papers as high as 26 mil. Since photo papers have resin coating, they do not need to be as thick as matte papers to resist warping from heavy ink coverage.