The raw material making up a sheet of paper varies somewhat between brands and types of paper, but a person can obtain a good estimate of a paper's thickness in millimeters (mm) from its density, which is expressed in grams per square meter (GSM). Since GSM is measured relative to area, the dimensions of a sheet of paper are irrelevant. If you know the density of a paper, you can visit a website which has a table giving thickness of paper in millimeters for various GSM values.
Navigate to a website with a table converting GSM to paper thickness. Micro Format, Inc. has compiled this information on its website.
Consult the appropriate table. On the Micro Format site, this is the second table on the page. Find the GSM value for your paper on the table. This is the right-hand column in this case, with a heading saying "Metric (grams/sq meter)". For example, if your paper is 90.3 GSM, look down the right column for this value.
Look across the table for the column showing paper thickness in metric units. In our example, we look one column to the left and find a column heading reading "millimeters". Read across the row for your paper's GSM value. For 90.3 GSM, we see the value of 0.12, which means our paper's thickness is approximately 0.12 millimeters.