Internationally, paper sizes are measured in millimetres. However, in the United States, paper size more commonly is measured in inches. A3 and A4 are two often-used international labels, and knowing what size these are in inches is crucial to choosing the correct paper for a printer or copier.
Based on the former Deutsche Industrie Norm (DIN), the International Standards Organization (ISO) assigns letters and numbers to paper based on metric size (A3 and A4 are two examples). ISO labels are commonly used in most parts of the world, but the United States more often refers to papers using standard English measurements, such as "8.5 x 11" (letter size) or "8.5 x 14" (legal size).
The ISO "A" classifications are used when ordering both inkjet and laser paper.
The classification A3 is assigned to paper that measures 297 x 420mm, or 11.69 x 16.54 inches. A3 is considered "functionally equivalent" to paper measuring 11 x 17 inches and is the size used for items such as twofold brochures, advertising documents and presentations.
The classification A4 is assigned to paper measuring 210 x 297mm, or 8.27 x 11.69 inches. A4 most closely matches letter-size paper, which measures 8.5 x 11 inches. A4 is used mostly for invoices, record-keeping and correspondence.
Based on the ISO international paper sizes, playing cards are classified as size A8 or B8, postcards are A6 and envelopes are either C4, C5 or C6.