In general, you measure it. In a few special cases you may be able to use geometry or trigonometry.
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Divide the length of the diagonal of a square by 1.4142 (which is the square root of 2) to find the length of a side. Similarly, to find the length of the diagonal of a square, multiply the length of a side by 1.4142.
If you know the length of the sides but not the diagonal, you can use the Pythagorean Theorem.
To find the length of the diagonal of a rectangle, you can use the Pythagorean theorem. In this case, the diagonal forms the hypotenuse of a right triangle with the sides being the length and width of the rectangle. So, the diagonal (d) can be calculated as d = √(4^2 + 6^2) = √(16 + 36) = √52 ≈ 7.21 feet.
If you know the length of the side, multiply it by the square root of 2.
Using Pythagoras: 322+362 = 2320 and the square root of this is the length of the diagonal