diff --git a/zipline/finance/blotter.py b/zipline/finance/blotter.py
index c3ad52d3..6f4bcfd2 100644
--- a/zipline/finance/blotter.py
+++ b/zipline/finance/blotter.py
@@ -261,3 +261,7 @@ class Order(object):
             return False
 
         return True
+
+    @property
+    def open_amount(self):
+        return self.amount - self.filled