diff --git a/conservancy/news/templatetags/fill_url.py b/conservancy/news/templatetags/fill_url.py
new file mode 100644
index 0000000000000000000000000000000000000000..e23d1c0fb77954af58540e324fcd863c81a794d7
--- /dev/null
+++ b/conservancy/news/templatetags/fill_url.py
@@ -0,0 +1,20 @@
+import urllib.parse
+
+from django import template
+
+register = template.Library()
+
+@register.filter(name='fill_url')
+def fill_url(given_url, base_url):
+    """"Fill out" missing pieces of one URL from another.
+
+    This function parses the given URL, and if it's missing any pieces
+    (scheme, netloc, etc.), it fills those in from the base URL.
+    Typical usage is "/URL/path"|fill_url:"https://hostname/"
+    to generate "https://hostname/URL/path".
+    """
+    given_parts = urllib.parse.urlsplit(given_url)
+    base_parts = urllib.parse.urlsplit(base_url)
+    return urllib.parse.urlunsplit(
+        given_part or base_part for given_part, base_part in zip(given_parts, base_parts)
+    )