Letter or ANSI Letter is a paper size standard defined by the American National Standards Institute, commonly used as home or office stationery in the United States, Canada, Chile , Haiti, Costa Rica, Mexico, Panama, Guatemala, the Dominican Republic and the Philippines. It measures 8.5 by 11 inches (215.9 by 279.4 mm), not similar to the A4 paper standard used by most other countries, defined in ISO 216 by the International Organization for Standardization.
The Reagan administration made Letter-size paper the norm for U.S. federal forms in the early 1980s; previously, the smaller "official" Government Letter size, 8 by 10.5 inches (203.2 by 266.7 mm) (aspect ratio: 1.3125), was used in government, while 8.5-by-11-inch (215.9 by 279.4 mm) paper was standard in most other offices. The aspect ratio is ?1.294 and the diagonal is ?13.901 () in length.
In the U.S., paper density is usually measured in "pounds per ream" (of 500 sheets). Typical Letter paper has a basis weight of paper of 20 or 24 pounds (9.1 or 10.9 kg) - the weight of 500 sheets (a ream) of 17-by-22-inch (431.8 by 558.8 mm) paper at 70 °F (21 °C) and at 50% humidity. One ream of 20-pound Letter-sized paper weighs 5 pounds (2.3 kg), and a single Letter-sized sheet of 20-pound paper weighs 0.16 ounces (4.536 g), which is equivalent to 75.19 g/m2. Some metric information is typically included on American ream packaging. For example, 20-pound paper is also labeled as 75 g/m2. The most common density of A4 paper is 80 g/m2.
The precise origins of the dimensions of US letter-size paper (8.5 × 11 in) are not known. The American Forest & Paper Association says that the standard US dimensions have their origin in the days of manual paper making, the 11" length of the standard paper being about a quarter of "the average maximum stretch of an experienced vatman's arms". The letter size falls within the range of the historical quarto size, which since pre-modern times refers to page sizes of 8 to 9 inches (200 to 230 mm) wide and 10 to 11 inches (250 to 280 mm) high, and it is indeed almost exactly one quarter of the old Imperial (British) paper size known as Demy 4to - by inches (440 by 570 mm) - allowing a inch (13 mm) for trimming.
The related paper size known as half letter, statement, or organizer L is exactly one half of the US Letter size: 8.5 by 5.5 inches (215.9 by 139.7 mm) (8.5 × 5.5 in).