diff --git a/lib/matplotlib/__init__.py b/lib/matplotlib/__init__.py
index 168e87ca2837..705e58b0a46f 100644
--- a/lib/matplotlib/__init__.py
+++ b/lib/matplotlib/__init__.py
@@ -887,6 +887,9 @@ def __iter__(self):
         """Yield sorted list of keys."""
         yield from sorted(dict.__iter__(self))
 
+    def __len__(self):
+        return dict.__len__(self)
+
     def find_all(self, pattern):
         """
         Return the subset of this RcParams dictionary whose keys match,