A1

The A1 paper size is the second largest of the A series formats defined in the ISO 216 standard. With dimensions of 594 mm x 841 mm, the A1 sheet has an area of 0.5 square meters, half the area of an A0 sheet. The A1 size was developed as part of the A series based on the aspect ratio of √2, enabling the scales of the paper sizes to neatly fold down.

The origins of A1 date back to the late 18th century in Germany along with the other A series sizes. The √2 aspect ratio was first proposed in a 1786 letter by German scientist Georg Christoph Lichtenberg. This geometry allowed for the creation of the A series with its ability to scale sizes up and down by folding sheets in half. The A1 format itself was formalized in the early 20th century German standards that led to the international standardization of the A series in 1975. Today, the A1 format is frequently used for large posters, charts and presentation slides. It also serves as a starting point to make booklets and other materials in smaller sizes like A3 and A4 when folded in half.