Never mind the paper sizes, the really crazy US measurement is the paper density: https://en.wikipedia.org/wiki/Paper_density https://en.wikipedia.org/wiki/Pape

Never mind the paper sizes, the really crazy US measurement is the paper density: https://en.wikipedia.org/wiki/Paper_density https://en.wikipedia.org/wiki/Paper_density tl;dr - most of the world uses grammage (g/m^2, ISO 536). The US uses pounds in a ream of paper in its uncut basis size. The basis size varies with the type of paper, and so does the ream size. It's a measurement that can only make sense to people who never take paper out of the box.