title: Radius of "viewable" region in google maps v3
body: <p>I need to know how to retrieve the radius of the viewable zoom level in google maps API v3.</p>
  
  <p>For example, if I am at zoom level 3, and depending on the users screen size (lets just say 400x400 viewable region) how do I get the "radius" circle of the viewable area. </p>
  
  <p>Alternatively I'm currently using map.fitBounds() for all the points I've added to the map so really all I <em>NEED</em> is the radius of all the bounds.  What I want is something like "20 miles" that I can feed into my database app.</p>