A5 format was standardized at the same time that all A sizes, in 1975 by ISO 216 standard. The latter comes from the German standard DIN 476, developed by Walter Portsmann in 1922. It referred to the explanations of Professor Lichtenberg on the issue, that were delivered on a sheet for the first time adopting a √2 ratio. This means that the length of a sheet must be equal to its width x 1.4142.

Thus, we obtain an homothetic system with a √2 ratio, which means that the A5 is obtained by folding an A4 in its width direction. This proportion is still preserved, which also keeps the contents of the sheet at the time of its enlargement.

The A5 has the following dimensions: 210 x 148 mm or 8.27 x 5.83 inches. Its printable surface is reduced when applying the usual print margins: 160 x 108 mm.

The surface of an A5 sheet is 0.031 m², or 0.04 square yard, 0.32 square foot, or 46.5 square inches. This size can still be used for writing, it also often comes in notebook.

The A5 is a dual A6, and an A7 quad. Conversely, it is half an A4 and quarter an A3. Calculating its weight is easy, knowing that it is 32 times smaller than A0 (or 1 m²). The weight of the paper being always applied in g/m², for a paper of 100 g/m², an A5 sheet is 100/32 = 3.125 grams.

- Square yard: 0.04
- Square foot: 0.32
- Square inch: 46.5

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