Foolscap is a size of paper, traditionally 8½ by 13½ inches (216 by 343 mm), though today it may be a bit smaller, and the term may be used loosely to refer to any large format paper. Technically, the measurements listed above define a size known as foolscap folio. Folio simply means half a sheet of paper, and foolscap is traditionally 17 by 13½ inches. Foolscap folio is usually referred to either as foolscap or as folio paper.

Foolscap has been a standard paper size since the 15th century, when it was first produced in Germany. It gets its name, typically pronounced full-scap or full-scape, from the watermark once used to identify it, in the shape of a jester's hat, or "fool's cap." Foolscap of the standard 17 by 13½ inches was divided into halves, quarters, and eighths to produce other standardized paper sizes, called folio, quarto, and octavio respectively.

Foolscap folio was the most widely used paper size throughout Europe and the British Commonwealth of Nations until the introduction of the ISO 216 standard in 1975. This standard defined many sizes of paper, of which A4 is most commonly used today. A4 paper, measuring 8.3 by 11.7 inches (210 by 297 mm) is considered standard for most of the world, with the notable exception of the United States, Canada, and Mexico, which typically use letter size, or 8½ by 11 inch (216 by 279 mm), paper.

Foolscap still exists, but is less common than it once was. It can sometimes be found at specialty stationers, and less commonly at office or school supply stores. The term foolscap is most often heard in England, Australia, and Canada, while in the United States, it is called legal size, which is standardized as 8½ by 14 inches (216 by 356 mm). Since foolscap is no longer commonly used, modern references to using it are often implicit of excessiveness or long-windedness.