CosmoCaixa Barcelona in the context of "Science museum"

Paper size refers to standardized dimensions for sheets of paper used globally in stationery, printing, and technical drawing. Most countries adhere to the ISO 216 standard, which includes the widely recognized A series (including A4 paper), defined by a consistent aspect ratio of √2. The system, first proposed in the 18th century and formalized in 1975, allows scaling between sizes without distortion. Regional variations exist, such as the North American paper sizes (e.g., Letter, Legal, and Ledger) which are governed by the ANSI and are used in North America and parts of Central and South America.

The standardization of paper sizes emerged from practical needs for efficiency. The ISO 216 system originated in late-18th-century Germany as DIN 476, later adopted internationally for its mathematical precision. The origins of North American sizes are lost in tradition and not well documented, although the Letter size (8.5 in × 11 in (216 mm × 279 mm)) became dominant in the US and Canada due to historical trade practices and governmental adoption in the 20th century. Other historical systems, such as the British Foolscap and Imperial sizes, have largely been phased out in favour of ISO or ANSI standards.