Card - GSM?

Card - A guide to weight (gsm) and caliper (microns)
Card thickness or caliper is measured in microns (1000 microns = 1 mm) but frequently sold or described by weight or gsm (grammes per square meter). Paper and Card is manufactured and sold by the metric tonne in Europe, so weight has always been the dominant format when specifying paper and card.
Card, or board to give it it's proper name is sold using both caliper and weight to give the buyer more information. Commercially available card for printing and craft use usually starts at around 200 microns and finishes at around 500 microns. This is about the limit for conventional paper machines, so 2 or more sheets of card are laminated together to make thicker card.
Lastly tradition throws another terminology for card thickness at us. Card used to be and sometimes still is described in terms of how many sheets of paper thick it is eg. a commercially available 230 micron card may be 180 gsm and known as '3-sheet'. I have summarized most popular card thicknesses below, showing commercially available gsm and micron figures