diff --git a/tests/test_bounding_box.py b/tests/test_bounding_box.py
new file mode 100644
index 000000000..cf7dccfb1
--- /dev/null
+++ b/tests/test_bounding_box.py
@@ -0,0 +1,31 @@
+from wkcuber.api.bounding_box import BoundingBox, Mag
+import pytest
+
+
+def test_align_with_mag():
+
+    assert BoundingBox((1, 1, 1), (10, 10, 10)).align_with_mag(Mag(2)) == BoundingBox(
+        topleft=(0, 0, 0), size=(12, 12, 12)
+    )
+    assert BoundingBox((1, 1, 1), (9, 9, 9)).align_with_mag(Mag(2)) == BoundingBox(
+        topleft=(0, 0, 0), size=(10, 10, 10)
+    )
+    assert BoundingBox((1, 1, 1), (9, 9, 9)).align_with_mag(Mag(4)) == BoundingBox(
+        topleft=(0, 0, 0), size=(12, 12, 12)
+    )
+    assert BoundingBox((1, 2, 3), (9, 9, 9)).align_with_mag(Mag(2)) == BoundingBox(
+        topleft=(0, 2, 2), size=(10, 10, 10)
+    )
+
+
+def test_in_mag():
+
+    with pytest.raises(AssertionError):
+        BoundingBox((1, 2, 3), (9, 9, 9)).in_mag(Mag(2))
+
+    with pytest.raises(AssertionError):
+        BoundingBox((2, 2, 2), (9, 9, 9)).in_mag(Mag(2))
+
+    assert BoundingBox((2, 2, 2), (10, 10, 10)).in_mag(Mag(2)) == BoundingBox(
+        topleft=(1, 1, 1), size=(5, 5, 5)
+    )
diff --git a/wkcuber/api/bounding_box.py b/wkcuber/api/bounding_box.py
index 8e5f21c17..f380560f2 100644
--- a/wkcuber/api/bounding_box.py
+++ b/wkcuber/api/bounding_box.py
@@ -185,20 +185,30 @@ def is_empty(self) -> bool:
 
         return not all(self.size > 0)
 
-    def in_mag(self, mag: Mag, ceil: bool = False) -> "BoundingBox":
+    def in_mag(self, mag: Mag) -> "BoundingBox":
 
         np_mag = np.array(mag.to_array())
 
-        def ceil_maybe(array: np.ndarray) -> np.ndarray:
-            if ceil:
-                return np.ceil(array)
-            return array
+        assert (
+            np.count_nonzero(self.topleft % np_mag) == 0
+        ), f"topleft {self.topleft} is not aligned with the mag {mag}. Use BoundingBox.align_with_mag()."
+        assert (
+            np.count_nonzero(self.bottomright % np_mag) == 0
+        ), f"bottomright {self.bottomright} is not aligned with the mag {mag}. Use BoundingBox.align_with_mag()."
 
         return BoundingBox(
-            topleft=ceil_maybe(self.topleft / np_mag).astype(np.int),
-            size=ceil_maybe(self.size / np_mag).astype(np.int),
+            topleft=(self.topleft // np_mag).astype(np.int),
+            size=(self.size // np_mag).astype(np.int),
         )
 
+    def align_with_mag(self, mag: Mag):
+        """Rounds the bounding box up, so that both topleft and bottomright are divisible by mag."""
+
+        np_mag = np.array(mag.to_array())
+        topleft = (self.topleft // np_mag).astype(np.int) * np_mag
+        bottomright = np.ceil(self.bottomright / np_mag).astype(np.int) * np_mag
+        return BoundingBox(topleft, bottomright - topleft)
+
     def contains(self, coord: Shape3D) -> bool:
 
         coord = np.array(coord)