Um is a measurement of the thickness of paper usually determined by a micrometer and expressed in microns. One micron is the equivalent of 1/1000 of a millimetre.
PUBLISHING TIP Often book designers and publishers confuse ums and GSM. GSM is not a measure of thickness but of paper weight. Due to differences in compaction, the same GSM paper can be thicker of thinner. While GSM is not a measure of thickness it is a good proxy measure. Designers should confirm spine width in mm with the printer especially for a paperback book.
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