On Tue, Jan 12, 2016 at 10:55 AM, Anna <[email protected]> wrote: > > 2016-01-11 22:26 GMT+01:00 Rob Hudson <[email protected]>: > > Can someone explain how lbs associate to gsm?: > > Basically they don't. The gsm are grams per square meter and therefore > give you the density of the paper. >
Technically this is not exactly density, but, having units of mass per unit area, is a property called "grammage": https://en.wikipedia.org/wiki/Grammage (Proper density would be mass per unit volume. There is, apparently, something called "area density" which exists for 2-dimensional objects... and paper is thin, but not quite THAT thin...) > The lbs are the pounds per ream. A ream is (most often but not always) > 500 standard-sized sheets, and different types of paper have different > standard sheet sizes. So this measurement unit is nuts, because it > tells you exactly nothing about the paper. This entry at Wikipedia: https://en.wikipedia.org/wiki/Paper#Types.2C_thickness_and_weight has some useful tidbits including reference to the oddness with US paper "weights": "In the United States, the weight assigned to a paper is the weight of a ream, 500 sheets, of varying "basic sizes", before the paper is cut into the size it is sold to end customers." And I have to agree with Anna, this is pretty crazy! I can't quite imagine why anyone let this go on for so long and didn't find a way to standardize. I suspect anyone in the trade just learns what the weights mean for all the different kinds of papers, and that if one needs to be absolutely unambiguous, one talks about grammage! The "Grammage" page goes into some more detail on it, and has a chart with the basis weights of different kinds of US papers, for reference. Anne
