diff --git a/src/calculator.py b/src/calculator.py
index 69ae964..6fe859f 100644
--- a/src/calculator.py
+++ b/src/calculator.py
@@ -1,6 +1,6 @@
 # retoor <retoor@molodetz.nl>
 
-from typing import Sequence
+from typing import Sequence, Union
 
 
 def add(left: int, right: int) -> int:
@@ -32,3 +32,10 @@ def variance(values: Sequence[float]) -> float:
     return sum((x - mean) ** 2 for x in values) / len(values)
 
 
+def percentage(value: Union[int, float], total: Union[int, float]) -> float:
+    if total == 0:
+        raise ValueError
+    return (value / total) * 100
+
+
+