A3 is a paper size that is typically used for drawings, diagrams and large tables. It is also frequently used in laser printers to output two A4 pages as a spread.
A3 measures 297 × 420 millimeters or 11.69 × 16.54 inches. In PostScript, its dimensions are rounded off to 842 × 1190 points.
Part of the ISO 216 standard
A3 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. A3 is actually often referred to as DIN A3. JIS P 0138-61 is the Japanese standard based on the same range of paper sizes.
By folding an A3 in two along its shortest side, you create a DIN A4 document. Two A3 pages next to each other in a spread equals the A2 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 A3 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 3508 x 4961 pixels. This is the required resolution for quality offset printing that will be viewed from a short distance (such as books, brochures, magazines, calendars,…). For photo books, it is also the optimum resolution but a somewhat lower pixel count (250 dpi) is acceptable for great-looking photographs.
- At 150 ppi the image needs to be 1754 x 2480 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 12 megapixels or more can be used to print high-quality A3 size pictures. Since pictures of this size are usually viewed from a somewhat larger distance an 8-megapixel camera will still offer excellent quality.
Other sources of information
An elaborate yet easy to read page on the ISO 216 standard can be found here.