A micron, a metric unit, equals one-millionth of a meter. Gsm stands for "grams per square meter." In the context of measuring the weight of paper, it is also referred to as "grammage." This measurement is not to be confused with the quantity in pounds on copy paper packaging, called "basis weight." Suppose you know how many microns thick a sheet of paper or posterboard is. To convert this micron count to gsm is a matter of multiplying by the paper's density, in the appropriate units.
Determine the mass and volume of large stack of the paper type of interest, preferably in grams and centimetres cubed. The easiest way to find the volume is to multiply the width and height from the label of the paper packaging by the thickness of the stack of paper. Measure the thickness yourself.
Find the paper's density by dividing the mass (in grams) by the volume (in cubic centimetres).
Convert the density into grams per meters-squared-microns by dividing the result in step 2 by 100. This makes sense because there are 100 times as many grams per meter as there are per centimetre, and a millionth as many grams per micron as per meter. 100x100/1,000,000 = 1/100.
Multiply the sheet thickness in microns by the conversion factor from Step 3 to get grams per square meter (gsm).