diff --git a/src/ifcopenshell-python/ifcopenshell/util/unit.py b/src/ifcopenshell-python/ifcopenshell/util/unit.py
index 7e4686d4e4..dfadebe5f1 100644
--- a/src/ifcopenshell-python/ifcopenshell/util/unit.py
+++ b/src/ifcopenshell-python/ifcopenshell/util/unit.py
@@ -644,6 +644,11 @@ def convert_unit(value: float, from_unit: ifcopenshell.entity_instance, to_unit:
     )
 
 
+def mm_to_m(value: float) -> float:
+    """Convert a millimetre value to metres."""
+    return value / 1000
+
+
 def convert(value: float, from_prefix: Optional[str], from_unit: str, to_prefix: Optional[str], to_unit: str) -> float:
     """Converts between length, area, and volume units
 
diff --git a/src/ifcopenshell-python/test/util/test_unit.py b/src/ifcopenshell-python/test/util/test_unit.py
index 5deef1884a..17f7fd9976 100644
--- a/src/ifcopenshell-python/test/util/test_unit.py
+++ b/src/ifcopenshell-python/test/util/test_unit.py
@@ -32,6 +32,17 @@ import test.bootstrap
 from ifcopenshell.util.shape_builder import ShapeBuilder
 
 
+class TestMmToM:
+    def test_converts_a_positive_value(self):
+        assert subject.mm_to_m(150) == 0.15
+
+    def test_returns_zero_for_zero(self):
+        assert subject.mm_to_m(0) == 0.0
+
+    def test_passes_through_negative_values(self):
+        assert subject.mm_to_m(-25) == -0.025
+
+
 class TestCacheUnits(test.bootstrap.IFC4):
     def test_run(self):
         ifcopenshell.api.root.create_entity(self.file, ifc_class="IfcProject")