How do you find the radius given the chord length?

If you are given a chord length of a circle, unless you are given more information about the chord, you can not determine what the radius of the circle will be.

This is because the chord length in a circle can vary from a length of (essentially) 0, up to a length of double the radius (the diameter).

The best you can say about the radius if given the chord length, is that the length of the radius is at least as long has half half the chord length.