diff --git a/zipline/protocol.py b/zipline/protocol.py
index f037b34c..cc3f0e59 100644
--- a/zipline/protocol.py
+++ b/zipline/protocol.py
@@ -181,3 +181,6 @@ class BarData(object):
 
     def items(self):
         return self._data.items()
+
+    def __len__(self):
+        return len(self._data.keys())