How is the length of a diagonal of a square found?

As a square has right angles, the diagonal forms a right triangle with two of the sides of the square.

Therefore use Pythagoras:

diagonal² = side² + side²

→ diagonal² = 2side²

→ diagonal = side × √2

Therefore to find the length of the diagonal of a square, multiply the side length of a square by the square root of 2.