A5

The A5 paper size is part of the smaller formats in the A series defined in the ISO 216 standard. With dimensions of 148 mm x 210 mm, the A5 sheet has an area of 0.03125 square meters. The A5 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 A5 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 A5 format itself was formalized in the early 20th century German standards that led to the international ISO standard published in 1975. The compact A5 size makes it popular for paperback books, notepads, and personal organizers. It also allows A5 booklets to be easily made from folding A4 sheets in half. The efficiency and portability of A5 paper contributes to its widespread use.