diff --git a/pylabrobot/visualizer3D/grids.py b/pylabrobot/visualizer3D/grids.py
index 46b84d5fd1b..b1d629cc36a 100644
--- a/pylabrobot/visualizer3D/grids.py
+++ b/pylabrobot/visualizer3D/grids.py
@@ -48,6 +48,17 @@ def describe_bands(resource: Resource) -> Optional[List[Dict[str, Any]]]:
   return described or None
 
 
+def _depth_in(child: Resource, frame: Resource) -> float:
+  """How far `child` reaches along `frame`'s y axis, in mm: its box, turned as it stands there."""
+  ys = [
+    child.get_location_wrt(frame, x, y, z).y
+    for x in ("l", "r")
+    for y in ("f", "b")
+    for z in ("b", "t")
+  ]
+  return max(ys) - min(ys)
+
+
 def describe_grid(resource: Resource) -> Optional[Dict[str, Any]]:
   """The repeated grid this resource lays positions on, or None if it lays none.
 
@@ -102,10 +113,9 @@ def describe_grid(resource: Resource) -> Optional[Dict[str, Any]]:
 
   # A mark runs to the back of what the positions carry, not of the resource: the deepest child
   # seated on the line (sharing its y), or the resource's own depth when nothing stands on it.
-  try:
-    depth = resource.get_absolute_size_y()
-  except Exception:
-    return None
+  # Both along the resource's own y, the frame the marks are drawn in, so turning the resource
+  # turns its marks without changing them.
+  depth = resource.get_size_y()
 
   seated: List[float] = []
   for child in getattr(resource, "children", []):
@@ -113,7 +123,7 @@ def describe_grid(resource: Resource) -> Optional[Dict[str, Any]]:
     if location is None or abs(location.y - first.y) > SEATED_TOLERANCE:
       continue
     try:
-      seated.append(child.get_absolute_size_y())
+      seated.append(_depth_in(child, resource))
     except Exception:
       continue
 
diff --git a/pylabrobot/visualizer3D/grids_tests.py b/pylabrobot/visualizer3D/grids_tests.py
new file mode 100644
index 00000000000..dcac972e2b5
--- /dev/null
+++ b/pylabrobot/visualizer3D/grids_tests.py
@@ -0,0 +1,30 @@
+"""The grids a resource lays its positions on, as the page is given them to draw."""
+
+import unittest
+
+from pylabrobot.resources.hamilton import STARLetDeck, hamilton_plate_carrier_L5_ac
+from pylabrobot.visualizer3D.grids import describe_grid
+
+
+class DescribeGridTests(unittest.TestCase):
+  """A deck's grid is measured in the deck's own frame, the frame its marks are drawn in."""
+
+  def test_a_turned_deck_has_the_grid_it_has_upright(self):
+    """The page draws a deck's marks in the deck's own frame and turns them with the deck, so the
+    grid is in that frame too. From a STARlet's front edge, its marks end at its back, 653.5 mm,
+    or at the back of a carrier on track 1, 560 mm: an `extent` of 590.5 or 497 mm from the first
+    track's line at 63 mm. A STARlet turned 90 degrees and measured along the world's y would end
+    its marks 1005 mm from its front edge, or 198 mm beside the carrier."""
+    for turn in (90, 37.5):
+      for with_carrier, extent in ((False, 590.5), (True, 497.0)):
+        upright, turned = STARLetDeck(), STARLetDeck()
+        if with_carrier:
+          for deck in (upright, turned):
+            deck.assign_child_resource(hamilton_plate_carrier_L5_ac("carrier"), track=1)
+        turned.rotate(z=turn)
+        with self.subTest(turn=turn, with_carrier=with_carrier):
+          upright_grid, turned_grid = describe_grid(upright), describe_grid(turned)
+          assert upright_grid is not None and turned_grid is not None
+          self.assertAlmostEqual(upright_grid.pop("extent"), extent, places=3)
+          self.assertAlmostEqual(turned_grid.pop("extent"), extent, places=3)
+          self.assertEqual(turned_grid, upright_grid)