International standard paper sizes

Standard paper sizes like ISO A4 are widely used all over the world today, with the USA being the notable exception. In the ISO paper size system, the height-to-width ratio of all pages is the square root of two (1.4142 : 1). In other words, the width and the height of a page relate to each other like the side and the diagonal of a square. This aspect ratio is especially convenient for a paper size. If you put two such pages next to each other, or equivalently cut one parallel to its shorter side into two equal pieces, then the resulting page will have again the same width/height ratio. As paper is usually specified in g/m², this simplifies calculation of the mass of a document if the format and number of pages are known.