diff --git a/neural_lam/models/ar_model.py b/neural_lam/models/ar_model.py
index 6dc8500b..1edb7931 100644
--- a/neural_lam/models/ar_model.py
+++ b/neural_lam/models/ar_model.py
@@ -129,12 +129,24 @@ def __init__(
                 persistent=False,
             )
 
+            # Compute dimensionalities (e.g. to instantiate MLPs)
             (
                 self.num_boundary_nodes,
                 boundary_static_dim,
             ) = self.boundary_static_features.shape
-            # TODO Compute boundary input dim separately
-            self.boundary_dim = self.grid_dim
+
+            # Compute boundary input dim separately
+            num_boundary_forcing_vars = datastore_boundary.get_num_data_vars(
+                category="forcing"
+            )
+            num_past_boundary_steps = args.num_past_boundary_steps
+            num_future_boundary_steps = args.num_future_boundary_steps
+            self.boundary_dim = (
+                boundary_static_dim
+                # Temporal Embedding counts as one additional forcing_feature
+                + (num_boundary_forcing_vars + 1)
+                * (num_past_boundary_steps + num_future_boundary_steps + 1)
+            )
 
         # Instantiate loss function
         self.loss = metrics.get_metric(args.loss)