diff --git a/docs/ref/templates/builtins.txt b/docs/ref/templates/builtins.txt
index 6e5e18543c..ba43e2eb15 100644
--- a/docs/ref/templates/builtins.txt
+++ b/docs/ref/templates/builtins.txt
@@ -1456,6 +1456,14 @@ For example::
 
 If ``value`` is 123456789, the output would be ``117.7 MB``.
 
+.. admonition:: File sizes and SI units
+
+    Strictly speaking, ``filesizeformat`` does not conform to the International
+    System of Units which recommends using KiB, MiB, GiB, etc. when byte sizes
+    are calculated in powers of 1024 (which is the case here). Instead, Django
+    uses traditional unit names (KB, MB, GB, etc.) corresponding to names that
+    are more commonly used.
+
 .. templatefilter:: first
 
 first