diff --git a/src/ol/extent.js b/src/ol/extent.js
index 746cecbda6..ae67c8a120 100644
--- a/src/ol/extent.js
+++ b/src/ol/extent.js
@@ -426,8 +426,10 @@ ol.extent.forEachCorner = function(extent, callback, opt_this) {
 
 
 /**
+ * Get the size of an extent.
  * @param {ol.Extent} extent Extent.
  * @return {number} Area.
+ * @api
  */
 ol.extent.getArea = function(extent) {
   var area = 0;