In Europe, paper is measured in GSM. This stands for ‘Grams per square metre’ and means exactly that ie how many grams a single sheet of paper weighs measuring 1m x 1m.
Common paper weights in cut sheets are:
A low quality sheet used in photo copiers is often 80gsm, where as a letterhead would commonly be printed on either 100gsm or 120gsm. Papers weighing either 130gsm or 150gsm are generally used for leaflets or flyers.
Anything weighing more than 170gsm is generally classified as board. At this weight a second measure can be used that indicates the thickness of the sheet. This measurement is given in microns where 1 micron = 1/1000 of a millimetre. This can be confusing as different sheets may weigh the same in GSM but have different thicknesses in microns. For example a piece of balsa wood and a piece of oak may be the same thickness but will weigh very different amounts. This is due to a number of factors during the manufacture of the board. These can include the type of coating on the board, the amount of virgin fibres used or bulkiness of the raw material.
Although paper is not quite as extreme, variations can occur between printed jobs that use different boards that might weigh the same but that have different thicknesses. This is most notable in uncoated boards with cheaper grades having the same thickness but weighing less than more superior grades.
Common board weights and their approximate thicknesses are:
Business cards are commonly 350gsm or 400gsm, postcards can vary from 250gsm to 350gsm and drop cards are often 300gsm.
As it is more common for paper and board to be sold in GSM it is easier to use this measure for both paper and board grades however if in doubt, samples can always be supplied.