There are two predominant paper size systems in use today: the international and North American systems. The international standard for paper size is ISO 216, which is based on the German DIN 476 standard for paper sizes. The most commonly used international paper size is A4, which measures 8.27 by 11.7 inches or 210 by 297 millimeters. The A series of paper sizes includes 11 sizes, designated A0–A10, all of which have an aspect ratio of 1:√2, where a is the long side and b is the short side. Since A series sizes share the same aspect ratio, they can be scaled to other A series sizes without being distorted, and two sheets can be reduced to fit on exactly one sheet without any cutoff or margins.

In North America, standard paper sizes are decided by the American National Standards Institute (ANSI) . The most common paper size in North America is Letter, which measures 8.5 by 11 inches or 216 by 279 millimeters. Legal size paper is also used in North America, which measures 8.5 by 14 inches or 216 by 356 millimeters.

In summary, the regular paper size depends on the region. In North America, the standard paper size is Letter, while in most other parts of the world, the most commonly used paper size is A4.