A1

A1 is a paper size that is typically used for technical drawings, flip charts and posters.

Dimensions

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.

Alternatives

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.