A9

The A9 paper size is an extra-small format in the A series in the ISO 216 standard. With minuscule dimensions of 37 mm x 52 mm, the A9 sheet has an area of just 0.001953125 square meters. The A9 size was developed as part of the A series based on the aspect ratio of √2, enabling each size to neatly fold down from the larger sheets.

The origins of A9 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 geometric ratio allowed for the creation of the A series with its ability to smoothly scale sizes up and down by folding sheets in half. The extremely tiny A9 format itself was formalized in the early 20th century German standards that led to the ISO standard published in 1975. The remarkably small A9 is suitable for tiny labels, stamps, and making micro booklets. Its minuscule dimensions allow multiple A9 pages to fit within larger A sizes. The incredibly compact nature of A9 paper makes it workable when space is extremely constrained.