diff --git a/src/ol/extent.js b/src/ol/extent.js
index 8e3ed2573a..1cf9d361a7 100644
--- a/src/ol/extent.js
+++ b/src/ol/extent.js
@@ -36,10 +36,7 @@ ol.Extent.boundingExtent = function(var_args) {
   var i;
   for (i = 1; i < arguments.length; ++i) {
     var coordinate = arguments[i];
-    extent.minX = Math.min(extent.minX, coordinate.x);
-    extent.minY = Math.min(extent.minY, coordinate.y);
-    extent.maxX = Math.max(extent.maxX, coordinate.x);
-    extent.maxY = Math.max(extent.maxY, coordinate.y);
+    extent.extendXY(coordinate.x, coordinate.y);
   }
   return extent;
 };
diff --git a/src/ol/rectangle.js b/src/ol/rectangle.js
index 70a4add53e..5763734ca2 100644
--- a/src/ol/rectangle.js
+++ b/src/ol/rectangle.js
@@ -59,6 +59,18 @@ ol.Rectangle.prototype.extend = function(rectangle) {
 };
 
 
+/**
+ * @param {number} x X.
+ * @param {number} y Y.
+ */
+ol.Rectangle.prototype.extendXY = function(x, y) {
+  this.minX = Math.min(this.minX, x);
+  this.minY = Math.min(this.minY, y);
+  this.maxX = Math.max(this.maxX, x);
+  this.maxY = Math.max(this.maxY, y);
+};
+
+
 /**
  * @return {ol.Coordinate} Center.
  */