In the U.S., the basis weight of paper is given in pounds per one ream (500 sheets) cut to the basic size (or trade size) for a specific grade.  Below, the common grades and trade sizes used in U.S. printing are given in the headings of the first four columns.  The basis weight is given down the column. The caliper column is a measurement of the thickness of paper, expressed in thousandths of an inch [mils]. The last column gives the metric weight (used by countries under the metric system). 

Use this chart to select two grades of paper that are similar weights by reading across a row.  Most U.S. paper distributors only handle a small portion of the weights noted.  U.S. printers normally carry a dozen or less paper weights.