diff --git a/gprMax/cmds_singleuse.py b/gprMax/cmds_singleuse.py
index ed722d37..ae451973 100644
--- a/gprMax/cmds_singleuse.py
+++ b/gprMax/cmds_singleuse.py
@@ -191,7 +191,7 @@ class TimeStepStabilityFactor(UserObjectSingle):
             raise ValueError
         
         G.dt_mod = f
-        G.dt = G.dt * G.dt_mod
+        G.calculate_dt()
 
         logger.info(f'Time step (modified): {G.dt:g} secs')
 
diff --git a/gprMax/grid.py b/gprMax/grid.py
index a4960727..069a8de2 100644
--- a/gprMax/grid.py
+++ b/gprMax/grid.py
@@ -46,7 +46,7 @@ class FDTDGrid:
         self.dy = 0
         self.dz = 0
         self.dt = 0
-        self.dt_mod = None # Time step stability factor
+        self.dt_mod = 1 # Time step stability factor
         self.iteration = 0 # Current iteration number
         self.iterations = 0 # Total number of iterations
         self.timewindow = 0
@@ -299,6 +299,9 @@ class FDTDGrid:
         # binary representation of floating point number.
         self.dt = round_value(self.dt, decimalplaces=d.getcontext().prec - 1)
 
+        # Apply any time step stability factor
+        self.dt = self.dt * self.dt_mod
+
 
 class CUDAGrid(FDTDGrid):
     """Additional grid methods for solving on GPU using CUDA."""
diff --git a/gprMax/subgrids/user_objects.py b/gprMax/subgrids/user_objects.py
index 18421be7..d4e12dbd 100644
--- a/gprMax/subgrids/user_objects.py
+++ b/gprMax/subgrids/user_objects.py
@@ -91,11 +91,8 @@ class SubGridBase(UserObjectMulti):
 
         self.set_discretisation(sg, grid)
 
-        # Set temporal discretisation including inheriting any time step 
-        # stability factor from the main grid
+        # Set temporal discretisation
         sg.calculate_dt()
-        if sg.dt_mod:
-            sg.dt = sg.dt * sg.dt_mod
 
         # Set the indices related to the subgrids main grid placement
         self.set_main_grid_indices(sg, uip, p1, p2)