diff --git a/maths/arc_length.py b/maths/arc_length.py
index 4c518f321dc7..2d0ea2e985c7 100644
--- a/maths/arc_length.py
+++ b/maths/arc_length.py
@@ -1,16 +1,16 @@
 from math import pi
 
 
-def arc_length(angle: int, radius: int) -> float:
+def arc_length(angle: int, diameter: int) -> float:
     """
     >>> arc_length(45, 5)
-    3.9269908169872414
+    1.9634954084936207
     >>> arc_length(120, 15)
-    31.415926535897928
+    15.707963267948964
     >>> arc_length(90, 10)
-    15.707963267948966
+    7.853981633974483
     """
-    return 2 * pi * radius * (angle / 360)
+    return pi * diameter * (angle / 360)
 
 
 if __name__ == "__main__":