Which is the aspect ratio of A4 paper?

The aspect ratio of all A series paper is:

This mean that if a is the bigger side then: a = 1.41 x b

Is the aspect ratio used by both A and B series. B series (B1, B2, B3…) is less common. It is bigger than A (B4 is 25.0 x 35.3 cm).

The length of the diagonal of an a4 paper is almost 36.3cm. This is 14.3 inches and we can say that one sheet is more or less the same as an 14″ laptop. But the ratio is nearest to 4:3 than to 16:9.