Is there any possible way of finding radius of the visible map from the middle point?

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edit: The following answer is for Google Maps JavaScript API v3
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I think the answer would be: Yes, you can.
According to the documentation, you can calculate distance between 2 points by: computeDistanceBetween(LatLngFrom, LatLngTo)
Also you can get the boundary of the map by using getBounds() method, which is in the google.maps.Map class.