diff --git a/SimPEG/Survey.py b/SimPEG/Survey.py
index e754476e..42ee50be 100644
--- a/SimPEG/Survey.py
+++ b/SimPEG/Survey.py
@@ -47,6 +47,13 @@ class BaseSurvey(object):
         self.prob._survey = None
         self._prob = None
 
+    @property
+    def nD(self):
+        """Number of data."""
+        if hasattr(self, 'dobs'):
+            return self.dobs.size
+        raise NotImplemented('Number of data is unknown.')
+
     @property
     def ispaired(self): return self.prob is not None