ISO 216
C Series  

C0  917 × 1297 
C1  648 × 917 
C2  458 × 648 
C3  324 × 458 
C4  229 × 324 
C5  162 × 229 
C6  114 × 162 
C7/6  81 × 162 
C7  81 × 114 
C8  57 × 81 
C9  40 × 57 
C10  28 × 40 
DL  110 × 220 
A Series  B Series  

A0  841 × 1189  B0  1000 × 1414  
A1  594 × 841  B1  707 × 1000  
A2  420 × 594  B2  500 × 707  
A3  297 × 420  B3  353 × 500  
A4  210 × 297  B4  250 × 353  
A5  148 × 210  B5  176 × 250  
A6  105 × 148  B6  125 × 176  
A7  74 × 105  B7  88 × 125  
A8  52 × 74  B8  62 × 88  
A9  37 × 52  B9  44 × 62  
A10  26 × 37  B10  31 × 44 
ISO 216 specifies international standard (ISO) paper sizes used in most countries in the world today, although not in Canada or the United States. The standard defines the "A" and "B" series of paper sizes, including A4, the most commonly available size. Two supplementary standards, ISO 217 and ISO 269, define related paper sizes; the ISO 269 "C" series is commonly listed alongside the A and B sizes.
All ISO 216, ISO 217 and ISO 269 paper sizes (except DL) have the same aspect ratio, 1:√2, at least to within the rounding to whole numbers of millimetres. This ratio has the unique property that when cut or folded in half widthwise, the halves also have the same aspect ratio. Each ISO paper size is one half of the area of the next larger size.
History
The advantages of basing a paper size upon an aspect ratio of √2 were already noted in 1786 by the German scientist Georg Christoph Lichtenberg, in a letter to Johann Beckmann.^{[1]} The formats that became A2, A3, B3, B4 and B5 were developed in France, and published in 1798 during the French Revolution.^{[2]}
Early in the twentieth century, Dr Walter Porstmann turned Lichtenberg's idea into a proper system of different paper sizes. Porstmann's system was introduced as a DIN standard (DIN 476) in Germany in 1922, replacing a vast variety of other paper formats. Even today the paper sizes are called "DIN Ax" in everyday use in Germany, Austria, Spain and Portugal.
The main advantage of this system is its scaling: if a sheet with an aspect ratio of √2 is divided into two equal halves parallel to its shortest sides, then the halves will again have an aspect ratio of √2. Folded brochures of any size can be made by using sheets of the next larger size, e.g. A4 sheets are folded to make A5 brochures. The system allows scaling without compromising the aspect ratio from one size to another – as provided by office photocopiers, e.g. enlarging A4 to A3 or reducing A3 to A4. Similarly, two sheets of A4 can be scaled down to fit exactly one A4 sheet without any cutoff or margins.
The weight of each sheet is also easy to calculate given the basis weight in grams per square metre (g/m^{2} or "gsm"). Since an A0 sheet has an area of 1 m^{2}, its weight in grams is the same as its basis weight in g/m^{2}. A standard A4 sheet made from 80 g/m^{2} paper weighs 5 g, as it is ^{1}⁄_{16} (four halvings, ignoring roundings to exact mm) of an A0 page. Thus the weight, and the associated postage rate, can be easily approximated by counting the number of sheets used.
ISO 216 and its related standards were first published between 1975 and 1995:
 ISO 216:2007, defining the A and B series of paper sizes
 ISO 269:1985, defining the C series for envelopes
 ISO 217:2013, defining the RA and SRA series of raw ("untrimmed") paper sizes
A series
Paper in the A series format has a 1:√2 ≈ 1.414 aspect ratio, although this is rounded to the nearest millimetre. A0 is defined so that it has an area of 1 square metre, prior to the rounding. Successive paper sizes in the series (A1, A2, A3, etc.) are defined by halving the preceding paper size, cutting parallel to its shorter side so that the long side of A(n + 1) is the same length as the short side of An prior to rounding.
The most frequently used of this series is the size A4 which is 210 mm × 297 mm (8.27 in × 11.7 in) and 1/16 square metres in area. For comparison, the letter paper size commonly used in North America (8.5 in × 11 in (216 mm × 279 mm)) is approximately 6 mm (0.24 in) wider and 18 mm (0.71 in) shorter than A4.
The geometric rationale behind the square root of 2 is to maintain the aspect ratio of each subsequent rectangle after cutting or folding an A series sheet in half, perpendicular to the larger side. Given a rectangle with a longer side, x, and a shorter side, y, ensuring that its aspect ratio, x/y, will be the same as that of a rectangle half its size, y/x/2, means that x/y = y/x/2, which reduces to x/y = √2; in other words, an aspect ratio of 1:√2.
The formula that gives the larger border of the paper size An in metres and without rounding off is the geometric sequence: . The paper size An thus has the dimension a_{n} × a_{n + 1} and area (prior to rounding off) 2^{−n} m^{2}.
The exact millimetre measurement of the long side of An is given by , in which the modified brackets represent the floor function.
B series
The B series is defined in the standard as follows: "A subsidiary series of sizes is obtained by placing the geometrical means between adjacent sizes of the A series in sequence." The use of the "geometric means", means that each step in size: B0, A0, B1, A1, B2 … is smaller than the previous by an equal scaling. As with the A series, the lengths of the B series have the ratio 1:√2, and folding one in half gives the next in the series. The shorter side of B0 is exactly 1 metre.
There is also an incompatible Japanese B series which the JIS defines to have 1.5 times the area of the corresponding JIS A series (which is identical to the ISO A series).^{[3]} Thus, the lengths of JIS B series paper are √1.5 ≈ 1.22 times those of Aseries paper. By comparison, the lengths of ISO B series paper are ^{4}√2 ≈ 1.19 times those of Aseries paper.
For the ISO B series, the exact millimetre measurement of the long side of Bn is given by .
C series
The C series formats are geometric means between the B series and A series formats with the same number (e.g., C2 is the geometric mean between B2 and A2). The width to height ratio is as in the A and B series. The C series formats are used mainly for envelopes. An A4 page will fit into a C4 envelope. C series envelopes follow the same ratio principle as the A series pages. For example, if an A4 page is folded in half so that it is A5 in size, it will fit into a C5 envelope (which will be the same size as a C4 envelope folded in half). The lengths of ISO C series paper are therefore ^{8}√2 times those of Aseries paper – i.e., about 9% larger.
A, B, and C paper fit together as part of a geometric progression, with ratio of successive side lengths of ^{8}√2, though there is no size halfway between Bn and A(n − 1): A4, C4, B4, "D4", A3, …; there is such a Dseries in the Swedish extensions to the system.
The exact millimetre measurement of the long side of Cn is given by .
Tolerances
The tolerances specified in the standard are:
 ±1.5 mm for dimensions up to 150 mm,
 ±2.0 mm for dimensions in the range 150 to 600 mm, and
 ±3.0 mm for dimensions above 600 mm.
A, B, C comparison
Size  A series formats  B series formats  C series formats  

(mm)  (in)  (mm)  (in)  (mm)  (in)  
0  841 × 1189  33.1 × 46.8  1000 × 1414  39.4 × 55.7  917 × 1297  36.1 × 51.1 
1  594 × 841  23.4 × 33.1  707 × 1000  27.8 × 39.4  648 × 917  25.5 × 36.1 
2  420 × 594  16.5 × 23.4  500 × 707  19.7 × 27.8  458 × 648  18.0 × 25.5 
3  297 × 420  11.7 × 16.5  353 × 500  13.9 × 19.7  324 × 458  12.8 × 18.0 
4  210 × 297  8.3 × 11.7  250 × 353  9.8 × 13.9  229 × 324  9.0 × 12.8 
5  148 × 210  5.8 × 8.3  176 × 250  6.9 × 9.8  162 × 229  6.4 × 9.0 
6  105 × 148  4.1 × 5.8  125 × 176  4.9 × 6.9  114 × 162  4.5 × 6.4 
7  74 × 105  2.9 × 4.1  88 × 125  3.5 × 4.9  81 × 114  3.2 × 4.5 
8  52 × 74  2.0 × 2.9  62 × 88  2.4 × 3.5  57 × 81  2.2 × 3.2 
9  37 × 52  1.5 × 2.0  44 × 62  1.7 × 2.4  40 × 57  1.6 × 2.2 
10  26 × 37  1.0 × 1.5  31 × 44  1.2 × 1.7  28 × 40  1.1 × 1.6 
Application
The ISO 216 formats are organized around the ratio 1:√2; two sheets next to each other together have the same ratio, sideways. In scaled photocopying, for example, two A4 sheets reduced to A5 size fit exactly onto one A4 sheet, and an A4 sheet in magnified size onto an A3 sheet; in each case, there is neither waste nor want.
The principal countries not generally using the ISO paper sizes are the United States and Canada, which use the Letter, Legal and Executive system. Although they have also officially adopted the ISO 216 paper format, Mexico, Panama, Venezuela, Colombia, the Philippines, and Chile also use mostly U.S. paper sizes.
Rectangular sheets of paper with the ratio 1:√2 are popular in paper folding, such as origami, where they are sometimes called "A4 rectangles" or "silver rectangles".^{[4]} In other contexts, the term "silver rectangle" can also refer to a rectangle in the proportion 1:(1 + √2), known as the silver ratio.
See also
 ANSI/ASME Y14.1
 International standard envelope sizes
 ISO 128 (relating to technical drawing)
 Letter (paper size)
 Metric Pixel Canvas
 Paper density
 Paper size
References
 ↑ Briefwechsel, Band, III; Lichtenberg (17861025). "Lichtenberg’s letter to Johann Beckmann". Georg Christoph Lichtenberg (in German) (1990 ed.). Deutschland: Verlag C. H. Beck. ISBN 3406309585. Retrieved 20090505.
 ↑ "Loi sur le timbre (Nº 2136)". Bulletin des lois de la République (in French) (Paris: French government) (237): 1–2. 17981103. Retrieved 20090505.
 ↑ "Japanese B Series Paper Size". Retrieved 20100418.
 ↑ Lister, David. "The A4 rectangle". The Lister List. England: British Origami Society. Retrieved 20090506.
External links
Wikimedia Commons has media related to DIN EN ISO 216. 
 International standard paper sizes: ISO 216 details and rationale
 ISO 216 at iso.org
