diff --git a/binance/helpers.py b/binance/helpers.py
index 8e02d0ea..75f0f190 100644
--- a/binance/helpers.py
+++ b/binance/helpers.py
@@ -1,5 +1,5 @@
 import asyncio
-from decimal import Decimal
+from decimal import Decimal, ROUND_DOWN
 import json
 from typing import Union, Optional, Dict
 
@@ -60,7 +60,9 @@ def interval_to_milliseconds(interval: str) -> Optional[int]:
 
 
 def round_step_size(
-    quantity: Union[float, Decimal], step_size: Union[float, Decimal]
+    quantity: Union[float, Decimal],
+    step_size: Union[float, Decimal],
+    rounding=ROUND_DOWN,
 ) -> float:
     """Rounds a given quantity to a specific step size
 
@@ -70,7 +72,8 @@ def round_step_size(
     :return: decimal
     """
     quantity = Decimal(str(quantity))
-    return float(quantity - quantity % Decimal(str(step_size)))
+    step_size = Decimal(str(step_size))
+    return float(quantity.quantize(step_size, rounding=rounding))
 
 
 def convert_ts_str(ts_str):
diff --git a/tests/test_helpers.py b/tests/test_helpers.py
new file mode 100644
index 00000000..a7b14262
--- /dev/null
+++ b/tests/test_helpers.py
@@ -0,0 +1,63 @@
+import pytest
+from decimal import Decimal, InvalidOperation
+from binance.helpers import round_step_size
+
+
+@pytest.mark.parametrize(
+    "quantity,step_size,expected",
+    [
+        # Basic cases
+        (1.23456, 0.1, 1.2),
+        (1.23456, 0.01, 1.23),
+        (1.23456, 1, 1),
+        # Edge cases
+        (0.0, 0.1, 0.0),
+        (0.1, 0.1, 0.1),
+        (1.0, 1, 1.0),
+        # Large numbers
+        (100.123456, 0.1, 100.1),
+        (1000.123456, 1, 1000),
+        # Small step sizes
+        (1.123456, 0.0001, 1.1234),
+        (1.123456, 0.00001, 1.12345),
+        # Decimal inputs
+        (Decimal("1.23456"), Decimal("0.1"), 1.2),
+        (Decimal("1.23456"), 0.01, 1.23),
+        # String conversion edge cases
+        (1.23456, Decimal("0.01"), 1.23),
+        ("1.23456", "0.01", 1.23),
+    ],
+)
+def test_round_step_size(quantity, step_size, expected):
+    """Test round_step_size with various inputs"""
+    result = round_step_size(quantity, step_size)
+    assert result == expected
+    assert isinstance(result, float)
+
+
+def test_round_step_size_precision():
+    """Test that rounding maintains proper precision"""
+    # Should maintain step size precision
+    assert round_step_size(1.123456, 0.0001) == 1.1234
+    assert round_step_size(1.123456, 0.001) == 1.123
+    assert round_step_size(1.123456, 0.01) == 1.12
+    assert round_step_size(1.123456, 0.1) == 1.1
+
+
+def test_round_step_size_always_rounds_down():
+    """Test that values are always rounded down"""
+    assert round_step_size(1.19, 0.1) == 1.1
+    assert round_step_size(1.99, 1.0) == 1.9
+    assert round_step_size(0.99999, 0.1) == 0.9
+
+
+def test_round_step_size_invalid_inputs():
+    """Test error handling for invalid inputs"""
+    with pytest.raises(InvalidOperation):
+        round_step_size(None, 0.1)  # type: ignore
+
+    with pytest.raises((ValueError, InvalidOperation)):
+        round_step_size("invalid", 0.1)  # type: ignore
+
+    with pytest.raises((ValueError, InvalidOperation)):
+        round_step_size(1.23, "invalid")  # type: ignore