ISO Paper Size Concept
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.
A Series Formats
A0 - 841 × 1189
A1 - 594 × 841
A2 - 420 × 594
SRA3 - 320 × 450
A3 - 297 × 420
A4 - 210 × 297
A5 - 148 × 210
A6 - 105 × 148
DLE - 99 x 210
Business Card - 55 x 90