diff --git a/src/ol/proj/proj.js b/src/ol/proj/proj.js
index 5c3fe43d3f..6f8d5112be 100644
--- a/src/ol/proj/proj.js
+++ b/src/ol/proj/proj.js
@@ -149,6 +149,7 @@ ol.proj.Projection.prototype.getUnits = function() {
  * Get the amount of meters per unit of this projection.  If the projection is
  * not configured with a units identifier, the return is `undefined`.
  * @return {number|undefined} Meters.
+ * @api
  */
 ol.proj.Projection.prototype.getMetersPerUnit = function() {
   return ol.proj.METERS_PER_UNIT[this.units_];