diff --git a/src/calibre/library/save_to_disk.py b/src/calibre/library/save_to_disk.py
index 537d7db7c1..95cb768c13 100644
--- a/src/calibre/library/save_to_disk.py
+++ b/src/calibre/library/save_to_disk.py
@@ -26,7 +26,7 @@ FORMAT_ARG_DESCS = dict(
         author_sort=_('The author sort string'),
         tags=_('The tags'),
         series=_('The series'),
-        series_index=_('The series number'),
+        series_index=_('The series number. To get leading zeros use {series_index:03n}'),
         rating=_('The rating'),
         isbn=_('The ISBN'),
         publisher=_('The publisher'),