The International Organization for Standardization (ISO) issues formal standards for measurements of almost everything that is used or made, including paper. Most of the world uses ISO "A format" paper sizes, based on the metric system, for administrative, commercial and technical writing and printed material, with important exceptions such as America and Canada. Companies must be familiar with and make allowances for the different paper sizes used in their overseas offices. Digital photographers also print on paper cut to ISO standards.
According to the ISO, in 1922 the Standards Committee of German Industry published the recommendation of a German engineer, Dr. Walter Porstmann, for a simple system of formatting paper sizes based on area. German corporations and government agencies soon adopted it and other countries followed. In 1961, the ISO recommended it as the world standard for trimmed paper sizes and printed matter. In 1975, the organisation formally adopted ISO 216, incorporating A and B formats of paper sizes.
The “A format” system
The "A format" series of paper sizes is based on area, beginning with A0, which has an area of one square metre (10.76 square feet). Each size is half the size of the previous one, so A1 paper has half the area of A0 and so forth. Also, as the Printernational website explains, the height and width of each ISO size are a ratio of the square root of two. Think of the width being the side of a square and the height being that square’s diagonal measurement.
A3, A4 and A5 are common ISO 216 sizes.
A3 paper is 29.7 by 42 cm (11.69 by 16.54 inches), most closely comparable to American “Tabloid” size, which is 27.9 by 43.2 cm (11 inches by 17 inches).
A4 is 21 by 29.7 cm (8.27 by 11.69 inches), a little narrower and a little longer than American “Letter” size, which is 21.6 by 27.9 cm (8.5 by 11 inches).
A5 dimensions are 14.8 by 21 cm (5.83 by 8.27 inches), between the American 12.7 by 20.3 cm (5 by 8 inch) index card and the 14 by 21.6 cm (5.5 by 8.5 inches) “Statement” size.
Two sheets of A5 paper make one A4 sheet. Two A4s put beside each other make one A3.
Corporations do business all over the world today. Printing materials on ISO standard paper sizes makes them acceptable anywhere. Digital photographers who print their own photos find that many professional photo papers for sale in the U.S. is sized by the ISO "A format" standards. Graphic designer Tom Hudgins says that’s because these paper sizes accommodate digital cameras’ sensor ratios well.
Buy A4 paper for laser or inkjet printers at major office supply outlets. Hudgins reports, "Paper companies supplying professionals offer custom-sizing services and will trim your selected paper to any A size you wish." Large corporate printing jobs done in China or elsewhere in Asia find A4 paper is the standard choice. Photo papers in glossy or matt finishes are available in "A format" sizes wherever sold.