A1 is a less commonly used standard of paper. The ISO 216 standard defines A1 size to be 841mm × 594mm, which is also exactly double the size of an A2.
Usually A1 sized paper is used for printing posters, advertisements, annual year planners etc. and is also a good size for architectural and technical drawings.
In the ISO paper size system, the height-to-width ratio of all pages is the square root of two. 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.
- A1 is twice the size of A2, and four times the size of A3
- A1 is eight times the size of A4,
- A1 is sixteen times the size of A5, and thirty two times the size of A6.
- A1 is half the size of A0.