diff --git a/humanfriendly/__init__.py b/humanfriendly/__init__.py
index 4c0a333..faea0e3 100644
--- a/humanfriendly/__init__.py
+++ b/humanfriendly/__init__.py
@@ -284,9 +284,13 @@ def format_length(num_metres, keep_width=False):
     >>> format_length(0.004)
     '4 mm'
     """
-    for unit in reversed(length_size_units):
+    units = list(reversed(length_size_units))
+    for index, unit in enumerate(units):
         if num_metres >= unit['divider']:
             number = round_number(float(num_metres) / unit['divider'], keep_width=keep_width)
+            if index and float(number) >= units[index - 1]['divider'] / unit['divider']:
+                unit = units[index - 1]
+                number = round_number(float(num_metres) / unit['divider'], keep_width=keep_width)
             return pluralize(number, unit['singular'], unit['plural'])
     return pluralize(num_metres, 'metre')
 
diff --git a/humanfriendly/tests.py b/humanfriendly/tests.py
index 72dad99..e61d09f 100644
--- a/humanfriendly/tests.py
+++ b/humanfriendly/tests.py
@@ -544,6 +544,7 @@ def test_format_length(self):
         self.assertEqual('1 metre', format_length(1))
         self.assertEqual('42 metres', format_length(42))
         self.assertEqual('1 km', format_length(1 * 1000))
+        self.assertEqual('1 km', format_length(999.999))
         self.assertEqual('15.3 cm', format_length(0.153))
         self.assertEqual('1 cm', format_length(1e-02))
         self.assertEqual('1 mm', format_length(1e-03))