diff --git a/src/sisl/physics/electron.py b/src/sisl/physics/electron.py
index f8c95e4a9..3a8295a1d 100644
--- a/src/sisl/physics/electron.py
+++ b/src/sisl/physics/electron.py
@@ -1576,7 +1576,7 @@ def berry_curvature(self, *args, **kwargs):
         return _berry_curvature(v, self.c)
 
     def effective_mass(self, *args, **kwargs):
-        r""" Calculate effective mass tensor for the states
+        r""" Calculate effective mass tensor for the states, units are (ps/Ang)^2
 
         This routine calls ``derivative(2, *args, **kwargs)`` and
         returns the effective mass for all states.