Haversine.js

var eartMeanRadiusInMeters = 6378137;

function rad(x) {
  return x * Math.PI / 180;
}

function distHaversine(p1, p2) {
  var radLat = rad(p2.lat() - p1.lat());
  var radLng = rad(p2.lng() - p1.lng());
  var intermediate =
    Math.sin(radLat / 2) * Math.sin(radLat / 2) +
    Math.cos(rad(p1.lat())) * Math.cos(rad(p2.lat())) * Math.sin(radLng / 2) * Math.sin(radLng / 2);
  return eartMeanRadiusInMeters * 2 * Math.atan2(Math.sqrt(intermediate), Math.sqrt(1 - intermediate));
}

// function getRadius() {
//   var currentBounds = map.getBounds(),
//     southWestOld = currentBounds.getSouthWest(),
//     northEastOld = currentBounds.getNorthEast(),
//     point1 = new google.maps.LatLng(northEastOld.lat(), southWestOld.lng()),
//     point2 = southWestOld;
//   return Math.ceil(distHaversine(point1, point2) / 2);
// }