diff --git a/src/validators/iban.py b/src/validators/iban.py
index da325ddb..ff39cd73 100644
--- a/src/validators/iban.py
+++ b/src/validators/iban.py
@@ -8,8 +8,8 @@
 
 
 def _char_value(char: str):
-    """A=10, B=11, ..., Z=35."""
-    return char if char.isdigit() else str(10 + ord(char) - ord("A"))
+    """A=10, B=11, ..., Z=35 (case-insensitive)."""
+    return char if char.isdigit() else str(10 + ord(char.upper()) - ord("A"))
 
 
 def _mod_check(value: str):
diff --git a/tests/test_iban.py b/tests/test_iban.py
index e3b0798f..329a20e0 100644
--- a/tests/test_iban.py
+++ b/tests/test_iban.py
@@ -13,6 +13,12 @@ def test_returns_true_on_valid_iban(value: str):
     assert iban(value)
 
 
+@pytest.mark.parametrize("value", ["gb82west12345698765432", "No9386011117947"])
+def test_returns_true_on_valid_lowercase_iban(value: str):
+    """Lowercase/mixed-case IBANs are accepted (the format regex is case-insensitive)."""
+    assert iban(value)
+
+
 @pytest.mark.parametrize("value", ["GB81WEST12345698765432", "NO9186011117947"])
 def test_returns_failed_validation_on_invalid_iban(value: str):
     """Test returns failed validation on invalid iban."""