"Radius" is a term most commonly associated with circles. When used in reference to a square, the radius is the distance from the center of the square to any of the four vertices, or where two sides intersect. Because a square has four right angles, you can find the diameter of the square using the Pythagorean Theorem.
Multiply the side length of the square by itself. For example, if each side of the square is six inches, you would multiply six times six to get 36 square inches.
Multiply the square of the side by two. In this example, multiply 36 by two to get 72 square inches.
Calculate the square root of the result found in step 2 to find the diameter of the square. In this example, take the square root of 72 to get 2.913 inches as the square's diameter.
Divide the square's diameter by two to find the radius of the square. Finishing the example, divide 2.913 inches by two to get 1.4565 inches, which is the radius of the square.