Letter (officiallyANSI A) is apaper size standard defined inANSI/ASME Y14.1 by theAmerican National Standards Institute, commonly used as home or office stationery primarily in theUnited States,Canada, and thePhilippines, and variably acrossLatin America.[1] It measures 8.5 by 11 inches (215.9 by 279.4 mm) and is similar in use to theA4 paper standard at 210 mm × 297 mm (8.27 in × 11.7 in) used by most other countries, defined inISO 216 by theInternational Organization for Standardization.
TheReagan administration made Letter-size paper the norm for US 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.[2] Theaspect ratio is22/17 ≈ 1.294 and thediagonal is√8.52 + 112 ≈ 13.901 inches (353.1 mm) in length.
In the US, paper density is usually measured in "pound perreams" (of 500 sheets). Typical Letter paper has abasis 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.[3] 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 related paper size known asInvoice (colloquially Half Letter) is exactly one half of the US Letter size: 8.5 by 5.5 inches (215.9 by 139.7 mm).
The precise origins of the dimensions of US letter-size paper (8.5 × 11 in) are not known. TheAmerican Forest & Paper Association says that the standard US dimensions have their origin in the days of manualpapermaking, the 11-inch length of the standard paper being about a quarter of "the average maximum stretch of an experiencedvatman's arms".[2] The letter size falls within the range of the historicalquarto 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 asdemy quarto –17+1⁄2 by22+1⁄2 inches (440 by 570 mm) – allowing a1⁄2 inch (13 mm) for trimming.[4]