### A4

In the A-series **paper size**, the ratio between length and width is √2 : 1.

##### Explanation

When a sheet of A4 paper is cut into two equal pieces (i.e. the surface halved), two sheets of A5 paper with exactly the same ratio in terms of dimensions are created.

√2 1

So it must apply

##### Note

If you turn a corner of a sheet of A4 paper, you get a diagonal with length √2. So that is exactly the length of the sheet.

√2 1

##### Other sizes

An A4 paper measures 297 mm by 210 mm. In the United States, the **US letter** format 279.4 mm by 215.8 mm is often used, which is 6 mm wider and 18 mm shorter.