diff --git a/src/calculate_average.py b/src/calculate_average.py
new file mode 100644
index 0000000..0b6b352
--- /dev/null
+++ b/src/calculate_average.py
@@ -0,0 +1,5 @@
+def calculate_average(values: list[float]) -> float:
+    """Return the arithmetic mean of values, or 0 if the list is empty."""
+    if not values:
+        return 0
+    return sum(values) / len(values)