How to set zoom to fit circle in Google Maps

A google.maps.Circle has a getBounds() method which returns the LatLngBounds of the circle. You may use this bounds as argument for google.maps.Map.fitBounds()

If using a circle, you can do this:

map.fitBounds(circle.getBounds());

...at the end of the init-function.

http://jsfiddle.net/doktormolle/MHLjy/

For multiple circles use union instead of extend:

var bounds = new google.maps.LatLngBounds();

$.each(circles, function(index, circle){
    bounds.union(circle.getBounds());
});

map.fitBounds(bounds);