diff --git a/src/calculator.py b/src/calculator.py
index e4e6afd..059ceb1 100644
--- a/src/calculator.py
+++ b/src/calculator.py
@@ -1,5 +1,8 @@
 # retoor <retoor@molodetz.nl>
 
+from typing import Union
+
+
 def add(left: int, right: int) -> int:
     return left + right
 
@@ -20,3 +23,11 @@ def clamp(value: int, low: int, high: int) -> int:
 
 def clamp_to_byte(value: int) -> int:
     return max(0, min(255, value))
+
+
+def percentage(value: Union[int, float], total: Union[int, float]) -> float:
+    if total == 0:
+        raise ValueError
+    return (value / total) * 100
+
+