diff --git a/docs/source/settings.rst b/docs/source/settings.rst
index 16d8961a..bf67bd25 100644
--- a/docs/source/settings.rst
+++ b/docs/source/settings.rst
@@ -1164,6 +1164,10 @@ Multiple addresses can be bound. ex.::
 will bind the `test:app` application on localhost both on ipv6
 and ipv4 interfaces.
 
+If the ``PORT`` environment variable is defined, the default
+is ``['0.0.0.0:$PORT']``. If it is not defined, the default
+is ``['127.0.0.1:8000']``.
+
 .. _backlog:
 
 backlog
diff --git a/gunicorn/config.py b/gunicorn/config.py
index 6c1d1777..d501db69 100644
--- a/gunicorn/config.py
+++ b/gunicorn/config.py
@@ -579,6 +579,10 @@ class Bind(Setting):
 
         will bind the `test:app` application on localhost both on ipv6
         and ipv4 interfaces.
+
+        If the ``PORT`` environment variable is defined, the default
+        is ``['0.0.0.0:$PORT']``. If it is not defined, the default
+        is ``['127.0.0.1:8000']``.
         """