A1 is a paper size that is typically used for technical drawings, flip charts and posters.
A1 measures 594 × 841 millimeters or 23.39 × 33.11 inch. In PostScript, its dimensions are rounded off to 1684 × 2384 points.
Part of the ISO 216 standard
A1 is part of a set or range of page sizes, called the ISO A or ISO 216 standard. This international standard is based on the German DIN 476 standard from 1922. A1 is actually often referred to as DIN A1. The Japanese have adopted the same range of paper sizes in their JIS P 0138-61 standard.
By folding an A1 in two along its shortest side, you create an A2 document. Two A1 pages next to each other in a spread equals the A0 paper size. This way a range of paper sizes is created from A0 (which has a surface of one square meter) to A10. The height/width ratio remains constant for all sizes: 1:1.41 or the square root of 2. The dimensions always get rounded to the nearest millimeter.
Resolution needed to print an A1 size picture
To output an image properly it needs to have a certain minimum resolution. The number of pixels depend on the required output quality.
- At 300 ppi (pixels per inch) the image needs to be 7016 x 9933 pixels. This is the required resolution for quality offset printing that will be viewed from a short distance (such as books, brochures, magazines, calenders,…). For photos it is also the optimum resolution but a somewhat lower pixel count (200 to 220 dpi) is acceptable for great looking photographs.
- At 150 ppi the image needs to be 3508 x 4967 pixels. This is the minimum resolution for newspapers or posters viewed from a fairly short distance.
A digital camera with decent image quality and a resolution of 20 megapixel or more can be used to print high quality A1 size pictures. Since such posters are usually viewed from a larger distance a 16 megapixel camera will still offer excellent quality and 12 megapixels will be considered good by most people.
The American alternative to A1 is the ANSI D-size, which is part of the ANSI/ASME Y14.1 standard. It measures 559 × 864 mm or 22 × 34″.
Other sources of information
An elaborate yet easy to read page on the ISO 216 standard can be found here.