A4 is the ISO 216 paper size measuring 210 by 297 millimetres.
A4 belongs to the A series of international paper sizes. Its proportions are based on the square root of two, so folding an A4 sheet in half along its longer side produces an A5 sheet while preserving the same shape.
The format is used for letters, worksheets, assignments, photocopies, and printer paper in much of the world. It is especially familiar during back-to-school preparation because many schools and offices use A4 folders, exercise sheets, and handouts.
A4 is often confused with US Letter, which measures 8.5 by 11 inches and is common in the United States and Canada. A3 is larger than A4, while A5 is smaller. The size name comes from the ISO series, not from the paper’s thickness or weight.