To calculate the thickness of a book or brochure we need to know the thickness of the paper rather than its weight. This is because the same weight paper will vary in thickness due to its bulk.
Paper thickness is measured in microns (a thousandth of a millimetre). As a rough guide, we can use an uncoated paper like Edixion Offset as an example; 90gsm is 114 microns, 120gsm is 143 microns, 170gsm is 187 microns and 250gsm is 275 microns.
For a more accurate calculation check with your printer, they will be able to give you the actual figure for the paper stock they will be using.