diff --git a/src/ifcopenshell-python/ifcopenshell/util/unit.py b/src/ifcopenshell-python/ifcopenshell/util/unit.py
index 20acca3e5b..e3c82b812b 100644
--- a/src/ifcopenshell-python/ifcopenshell/util/unit.py
+++ b/src/ifcopenshell-python/ifcopenshell/util/unit.py
@@ -466,7 +466,16 @@ def cache_units(ifc_file: ifcopenshell.file) -> None:
     """
     ifc_file.units = {}
     if assignment := get_unit_assignment(ifc_file):
-        ifc_file.units = {u.UnitType: u for u in assignment.Units if getattr(u, "UnitType", None)}
+        all_units = assignment.Units or []
+        units = {u.UnitType: u for u in all_units if getattr(u, "UnitType", None)}
+        # As in get_project_unit(): a literal match always wins; an IfcDerivedUnit with no
+        # literal UnitType match is matched dimensionally instead, only to fill a gap.
+        for unit in all_units:
+            if unit.is_a("IfcDerivedUnit"):
+                dimension = identify_unit_dimensions(unit)
+                if dimension and dimension not in units:
+                    units[dimension] = unit
+        ifc_file.units = units
 
 
 def clear_unit_cache(ifc_file: ifcopenshell.file) -> None:
@@ -482,6 +491,10 @@ def get_project_unit(
 ) -> Union[ifcopenshell.entity_instance, None]:
     """Get the default project unit of a particular unit type
 
+    IfcDerivedUnit is matched first by a literal `UnitType` match, then, as a fallback, by
+    dimensional analysis (:func:`identify_unit_dimensions`), mirroring
+    :func:`get_candidate_units`.
+
     :param ifc_file: The IFC file.
     :param unit_type: The type of unit, taken from the list of IFC unit types,
         such as "LENGTHUNIT".
@@ -493,9 +506,13 @@ def get_project_unit(
     if units := ifc_file.units:
         return units.get(unit_type, None)
     if unit_assignment := get_unit_assignment(ifc_file):
+        dimensional_match = None
         for unit in unit_assignment.Units or []:
             if getattr(unit, "UnitType", None) == unit_type:
                 return unit
+            if dimensional_match is None and unit.is_a("IfcDerivedUnit") and identify_unit_dimensions(unit) == unit_type:
+                dimensional_match = unit
+        return dimensional_match
 
 
 def get_candidate_units(ifc_file: ifcopenshell.file, unit_type: str) -> list[ifcopenshell.entity_instance]:
diff --git a/src/ifcopenshell-python/test/util/test_unit.py b/src/ifcopenshell-python/test/util/test_unit.py
index dd304fd77b..26daad34b6 100644
--- a/src/ifcopenshell-python/test/util/test_unit.py
+++ b/src/ifcopenshell-python/test/util/test_unit.py
@@ -94,6 +94,37 @@ class TestGetProjectUnit(test.bootstrap.IFC4):
         assert subject.get_project_unit(self.file, "LENGTHUNIT", use_cache=True) == length2
         assert self.file.units == {"LENGTHUNIT": length2, "AREAUNIT": area}
 
+    def test_area_and_volume_derived_from_length_are_matched_dimensionally(self):
+        # AREAUNIT/VOLUMEUNIT have no IfcDerivedUnitEnum member, so a project whose area/volume
+        # default is an IfcDerivedUnit has no literal UnitType match for either -- get_project_unit
+        # must still resolve them by dimensional analysis, like get_candidate_units already does.
+        ifcopenshell.api.root.create_entity(self.file, ifc_class="IfcProject")
+        length = ifcopenshell.api.unit.add_si_unit(self.file, unit_type="LENGTHUNIT")
+        area = ifcopenshell.api.unit.add_derived_unit(self.file, "USERDEFINED", "area-ish", {length: 2})
+        volume = ifcopenshell.api.unit.add_derived_unit(self.file, "USERDEFINED", "volume-ish", {length: 3})
+        ifcopenshell.api.unit.assign_unit(self.file, units=[length, area, volume])
+
+        assert subject.get_project_unit(self.file, "AREAUNIT") == area
+        assert subject.get_project_unit(self.file, "VOLUMEUNIT") == volume
+
+    def test_literal_unit_type_match_takes_priority_over_dimensional(self):
+        ifcopenshell.api.root.create_entity(self.file, ifc_class="IfcProject")
+        length = ifcopenshell.api.unit.add_si_unit(self.file, unit_type="LENGTHUNIT")
+        literal_area = ifcopenshell.api.unit.add_si_unit(self.file, unit_type="AREAUNIT")
+        derived_area = ifcopenshell.api.unit.add_derived_unit(self.file, "USERDEFINED", "area-ish", {length: 2})
+        ifcopenshell.api.unit.assign_unit(self.file, units=[length, literal_area, derived_area])
+
+        assert subject.get_project_unit(self.file, "AREAUNIT") == literal_area
+
+    def test_dimensional_fallback_also_applies_when_using_a_cache(self):
+        ifcopenshell.api.root.create_entity(self.file, ifc_class="IfcProject")
+        length = ifcopenshell.api.unit.add_si_unit(self.file, unit_type="LENGTHUNIT")
+        area = ifcopenshell.api.unit.add_derived_unit(self.file, "USERDEFINED", "area-ish", {length: 2})
+        ifcopenshell.api.unit.assign_unit(self.file, units=[length, area])
+
+        assert subject.get_project_unit(self.file, "AREAUNIT", use_cache=True) == area
+        assert self.file.units["AREAUNIT"] == area
+
 
 class TestGetCandidateUnits(test.bootstrap.IFC4):
     def test_returns_only_units_matching_the_unit_type(self):