There is no general formula. You could use basic trigonometry.
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You measure it. If you know the lengths of two other sides, as well as the angle between them, you can also use the law of cosines.
If they are the dimensions of a rectangle then use Pythagoras' theorem to find the length of the diagonal.
The square has a diagonal measurement of: 1.13 km
Two of them
A quadrilateral in which diagonal are not congruent and only larger diagonal is perpendicular bisector of smaller diagonal then it is known as kite -- Mohan S. Vighe
A diagonal always forms an angle bisector in a square. In a rectangle, trapezoid, or any other quadrilateral, a diagonal does not always bisect the angles.