Foolscap Folio (commonly contracted to foolscap or folio) is paper cut to the size of 8½ × 13½ inches (216 × 342 mm). This was a traditional paper size used in Europe and the British Commonwealth, before the adoption of modern international paper sizes there. It remains a common size for ring binders/lever arch files containing A4 paper (the most common standard size outside the United States), because it offers greater protection to the edge of the pages than an A4 binder.

Recently it has met more widespread use in some circles in the United States, where the term is applied to any paper measuring about 8½ × 13 inches (216 × 330 mm).

A full foolscap paper sheet is 17 x 13½ inches (432 × 342 mm) in size, and a folio sheet of any type is simply half the standard sheet size.

Foolscap was named after the fool's cap and bells watermark commonly used from the fifteenth century onwards on paper measuring 17 × 13½ inches (432 × 342 mm) or a subdivision of this into halves, quarters and so on. The earliest example of such paper that is firmly dated was made in Germany in 1479.

Unsubtantiated anecodotes suggest that the watermark was introduced to England in 1580 by Sir John Spielmann , a German who established a papermill at Dartford, Kent. Apocryphally, the Rump Parliament substituted a fools cap for the royal arms as a watermark on the paper used for the journals of parliament.

In Brazil that paper size is also sometimes called Ofício II, a reference to the 8½ × 14 inches (216 × 356 mm) paper size (which is named Legal but in Portuguese is better known as Ofício).