Caliper
Paper thickness, or caliper, is a common measurement specified and required for certain printing applications. Since a paper's density is typically not directly known or specified, the thickness of any sheet of paper cannot be calculated by any method. Instead, it is measured and specified separately as its caliper. However, paper thickness for most typical business papers might be similar across comparable brands. If thickness is not specified for a paper in question, it must be either measured or guessed based on a comparable paper's specification.
Caliper is usually measured in micrometres (1/1000 of a mm), or in the United States also in mils. (1 mil = 0.001 inch = 25.4 µm) In common measure "20-lb bond varies from about 0.0038 to 0.0045 in. thick."
Read more about this topic: Paper Density