diff --git a/jedi/api/classes.py b/jedi/api/classes.py
index a8d9fb19..42aac726 100644
--- a/jedi/api/classes.py
+++ b/jedi/api/classes.py
@@ -749,6 +749,24 @@ class Completion(BaseName):
 
         return super().type
 
+    def get_completion_prefix_length(self):
+        """
+        Returns the length of the prefix being completed.
+        For example, completing ``isinstance``::
+
+            isinstan# <-- Cursor is here
+
+        would return 8, because len('isinstan') == 8.
+
+        Assuming the following function definition::
+
+            def foo(param=0):
+                pass
+
+        completing ``foo(par`` would return 3.
+        """
+        return self._like_name_length
+
     def __repr__(self):
         return '<%s: %s>' % (type(self).__name__, self._name.get_public_name())
 
diff --git a/test/test_api/test_classes.py b/test/test_api/test_classes.py
index 6add83f6..4a6f7323 100644
--- a/test/test_api/test_classes.py
+++ b/test/test_api/test_classes.py
@@ -513,10 +513,14 @@ def test_added_equals_to_params(Script):
 
     assert run('foo(bar').name_with_symbols == 'bar='
     assert run('foo(bar').complete == '='
+    assert run('foo(bar').get_completion_prefix_length() == 3
     assert run('foo(bar, baz').complete == '='
+    assert run('foo(bar, baz').get_completion_prefix_length() == 3
     assert run('    bar').name_with_symbols == 'bar'
     assert run('    bar').complete == ''
+    assert run('    bar').get_completion_prefix_length() == 3
     x = run('foo(bar=isins').name_with_symbols
+    assert run('foo(bar=isins').get_completion_prefix_length() == 5
     assert x == 'isinstance'