diff --git a/tests/unit/test_hardware.py b/tests/unit/test_hardware.py
index 98a544a237..341be10db4 100644
--- a/tests/unit/test_hardware.py
+++ b/tests/unit/test_hardware.py
@@ -67,6 +67,26 @@ def test_constraint_name_pattern(value: str, expected: tuple[Any, Any]) -> None:
     assert match.groups() == expected
 
 
+_size_constraint_pattern_input = [
+    ({'name': 'num_with_default', 'raw_value': '10', 'default_unit': 'GiB'},
+     'num_with_default: == 10 gibibyte'),
+    ({'name': 'num_without_default', 'raw_value': '1024'}, 'num_without_default: == 1024 byte'),
+    ({'name': 'num_with_unit', 'raw_value': '10 GiB', 'default_unit': 'MiB'},
+     'num_with_unit: == 10 GiB'),
+    ]
+
+
+@pytest.mark.parametrize(
+    ('value', 'expected'),
+    _size_constraint_pattern_input,
+    )
+def test_constraint_default_unit(value: dict, expected: tuple[Any, Any]) -> None:
+    constraint_out = tmt.hardware.SizeConstraint.from_specification(**value)
+
+    assert constraint_out is not None
+    assert str(constraint_out) == expected
+
+
 _constraint_components_pattern_input = [
     ('memory 10 GiB', ('memory', None, None, None, '10 GiB')),
     ('cpu.processors != 4 ', ('cpu', None, 'processors', '!=', '4')),
diff --git a/tmt/hardware.py b/tmt/hardware.py
index 43fae1edd6..deb40f8748 100644
--- a/tmt/hardware.py
+++ b/tmt/hardware.py
@@ -493,7 +493,7 @@ class Constraint(BaseConstraint, Generic[ConstraintValueT]):
     raw_value: str
 
     # If set, it is a raw unit specified by the constraint.
-    unit: Optional[str] = None
+    default_unit: Optional[str] = None
 
     # If set, it is a "bigger" constraint, to which this constraint logically
     # belongs as one of its aspects.
@@ -507,7 +507,8 @@ def _from_specification(
             as_quantity: bool = True,
             as_cast: Optional[Callable[[str], ConstraintValueT]] = None,
             original_constraint: Optional['Constraint[Any]'] = None,
-            allowed_operators: Optional[list[Operator]] = None
+            allowed_operators: Optional[list[Operator]] = None,
+            default_unit: Optional[Any] = "bytes"
             ) -> T:
         """
         Parse raw constraint specification into our internal representation.
@@ -520,6 +521,7 @@ def _from_specification(
         :param original_constraint: when specified, new constraint logically belongs to
             ``original_constraint``, possibly representing one of its aspects.
         :param allowed_operators: if specified, only operators on this list are accepted.
+        :param default_unit: if raw_value contains no unit, this unit will be appended.
         :raises ParseError: when parsing fails, or the operator is now allowed.
         :returns: a :py:class:`Constraint` representing the given specification.
         """
@@ -560,7 +562,7 @@ def _from_specification(
             if not isinstance(
                     value,
                     pint.Quantity):  # type: ignore[reportUnnecessaryIsInstance,unused-ignore]
-                value = pint.Quantity(value)
+                value = pint.Quantity(value, default_unit)
 
         elif as_cast is not None:
             value = as_cast(raw_value)
@@ -672,14 +674,16 @@ def from_specification(
             name: str,
             raw_value: str,
             original_constraint: Optional['Constraint[Any]'] = None,
-            allowed_operators: Optional[list[Operator]] = None
+            allowed_operators: Optional[list[Operator]] = None,
+            default_unit: Optional[Any] = 'bytes'
             ) -> T:
         return cls._from_specification(
             name,
             raw_value,
             as_quantity=True,
             original_constraint=original_constraint,
-            allowed_operators=allowed_operators
+            allowed_operators=allowed_operators,
+            default_unit=default_unit
             )