diff --git a/src/pf1e_simulator/map.py b/src/pf1e_simulator/map.py
index dcf87c9..1adc4f4 100644
--- a/src/pf1e_simulator/map.py
+++ b/src/pf1e_simulator/map.py
@@ -20,6 +20,12 @@ Pos = tuple[int, int]  # (row, col), 0-indexed
 
 _ELEVATION_CHARS = frozenset("0123456789.")  # digit = height, '.' = ground level
 
+# PF1e (SRD, Measuring Distance): one grid square is 5 feet. The engine's
+# whole geometry (5-10-5 diagonals, step costs, ranges, auras) is hard-coded
+# to that scale, so a map declaring any other square size is a data error --
+# fail fast at load instead of silently mis-scaling.
+_SQUARE_SIZE_FT = 5
+
 _COVER_BONUS_LEVELS = frozenset({0, 2, 4, 8})  # PF1e: none / partial / standard / improved
 
 
@@ -152,6 +158,14 @@ def load_map(path: Path) -> MapSpec:
     except ValidationError as exc:
         raise MapValidationError(path, str(exc)) from exc
 
+    if raw.square_size_ft != _SQUARE_SIZE_FT:
+        msg = (
+            f"square_size_ft must be {_SQUARE_SIZE_FT} (one grid square is 5 "
+            "feet, SRD Measuring Distance); the engine geometry is hard-coded "
+            f"to 5-ft squares, got {raw.square_size_ft}"
+        )
+        raise MapValidationError(path, msg)
+
     terrain = _split_rows(raw.terrain, "terrain", path)
     height, width = len(terrain), len(terrain[0])
     unknown = sorted({ch for row in terrain for ch in row if ch not in raw.legend})
diff --git a/tests/test_map.py b/tests/test_map.py
index b10eef9..d1f50f2 100644
--- a/tests/test_map.py
+++ b/tests/test_map.py
@@ -73,6 +73,20 @@ def test_rejects_ragged_rows(tmp_path: Path) -> None:
         load_map(_write(tmp_path, body))
 
 
+def test_rejects_non_five_foot_square_size(tmp_path: Path) -> None:
+    """SRD Measuring Distance: one grid square is 5 ft; the engine geometry
+    is hard-coded to that scale, so any other declared size is a load error
+    (fail fast) rather than a silent mis-scaling."""
+    body = _minimal_map().replace('name: "Test"\n', 'name: "Test"\nsquare_size_ft: 10\n')
+    with pytest.raises(MapValidationError, match="square_size_ft must be 5"):
+        load_map(_write(tmp_path, body))
+
+
+def test_accepts_explicit_five_foot_square_size(tmp_path: Path) -> None:
+    body = _minimal_map().replace('name: "Test"\n', 'name: "Test"\nsquare_size_ft: 5\n')
+    assert load_map(_write(tmp_path, body)).square_size_ft == 5
+
+
 def test_rejects_char_missing_from_legend(tmp_path: Path) -> None:
     body = _minimal_map().replace("  #.#\n", "  #x#\n")
     with pytest.raises(MapValidationError):