diff --git a/boxtree/tree_build.py b/boxtree/tree_build.py
index b03c336e53205b5e61e2c91ce7c57610aeb295f3..fcbf1dc47bf66069514e3b0564a17051903002c9 100644
--- a/boxtree/tree_build.py
+++ b/boxtree/tree_build.py
@@ -458,7 +458,9 @@ class TreeBuilder(object):
                 queue, box_parent_ids.data, np.zeros((), dtype=box_parent_ids.dtype))
         prep_events.append(evt)
 
-        nlevels_max = np.finfo(coord_dtype).nmant + 1  # num bits in the significand
+        # 2*(num bits in the significand)
+        # https://gitlab.tiker.net/inducer/boxtree/issues/23
+        nlevels_max = 2*(np.finfo(coord_dtype).nmant + 1)
         assert nlevels_max <= np.iinfo(self.box_level_dtype).max
 
         # level -> starting box on level