diff --git a/openmc/deplete/operator.py b/openmc/deplete/operator.py
index e3a767e574..d8aeb33fd2 100644
--- a/openmc/deplete/operator.py
+++ b/openmc/deplete/operator.py
@@ -341,7 +341,7 @@ class Operator(TransportOperator):
                 if mat.volume is None:
                     raise RuntimeError("Volume not specified for depletable "
                                        "material with ID={}.".format(mat.id))
-                volume[str(mat.id)] = mat.volume
+                volume[str(mat.id)] = mat.volume/mat.num_instances
                 self.heavy_metal += mat.fissionable_mass
 
         # Make sure there are burnable materials