diff --git a/src/ifc5d/ifc5d/ifc5Dspreadsheet.py b/src/ifc5d/ifc5d/ifc5Dspreadsheet.py
index a0f12e2535..a57e52426c 100644
--- a/src/ifc5d/ifc5d/ifc5Dspreadsheet.py
+++ b/src/ifc5d/ifc5d/ifc5Dspreadsheet.py
@@ -269,15 +269,18 @@ class IfcDataGetter:
                                 if obj.is_a("IfcElement"):
                                     prefix += (obj.Name or "") + " - "
             name = prefix + (quantity.Name or "")
+            # Formula is an optional IfcLabel on IfcQuantity* in IFC4+; absent in
+            # IFC2X3, hence the schema-safe getattr.
+            formula = getattr(quantity, "Formula", None) or ""
             if quantity.is_a("IfcPhysicalSimpleQuantity"):
                 value = quantity[3]
                 try:
                     value = float(value) if value is not None else 0.0
                 except (TypeError, ValueError):
                     value = 0.0
-                result.append([name, value])
+                result.append([name, value, formula])
             else:
-                result.append([name + " ERROR: Only IfcPhysicalSimpleQuantity is supported", 0.0])
+                result.append([name + " ERROR: Only IfcPhysicalSimpleQuantity is supported", 0.0, formula])
         return json.dumps(result, ensure_ascii=False)
 
 
diff --git a/src/ifc5d/test/test_csv2ifc.py b/src/ifc5d/test/test_csv2ifc.py
index a630bb44ac..01b4a85ee6 100644
--- a/src/ifc5d/test/test_csv2ifc.py
+++ b/src/ifc5d/test/test_csv2ifc.py
@@ -130,7 +130,7 @@ class TestSerialiseCostQuantities:
 
         result = ifc5d.ifc5Dspreadsheet.IfcDataGetter.serialise_cost_quantities(ifc_file, cost_item)
 
-        assert json.loads(result) == [[name, 12.5]]
+        assert json.loads(result) == [[name, 12.5, ""]]
 
     def test_unset_name_does_not_crash(self):
         ifc_file = ifcopenshell.file()
@@ -140,4 +140,23 @@ class TestSerialiseCostQuantities:
 
         result = ifc5d.ifc5Dspreadsheet.IfcDataGetter.serialise_cost_quantities(ifc_file, cost_item)
 
-        assert json.loads(result) == [["", 3.0]]
+        assert json.loads(result) == [["", 3.0, ""]]
+
+    def test_formula_is_included_when_present(self):
+        ifc_file = ifcopenshell.file()
+        quantity = ifc_file.create_entity("IfcQuantityArea", Name="Area", AreaValue=12.5, Formula="Length * Width")
+        cost_item = ifc_file.create_entity("IfcCostItem", CostQuantities=[quantity])
+
+        result = ifc5d.ifc5Dspreadsheet.IfcDataGetter.serialise_cost_quantities(ifc_file, cost_item)
+
+        assert json.loads(result) == [["Area", 12.5, "Length * Width"]]
+
+    def test_quantity_without_formula_attribute_does_not_crash(self):
+        # IfcPhysicalComplexQuantity has no Formula attribute and is unsupported.
+        ifc_file = ifcopenshell.file()
+        quantity = ifc_file.create_entity("IfcPhysicalComplexQuantity", Name="Complex", Discrimination="layer")
+        cost_item = ifc_file.create_entity("IfcCostItem", CostQuantities=[quantity])
+
+        result = ifc5d.ifc5Dspreadsheet.IfcDataGetter.serialise_cost_quantities(ifc_file, cost_item)
+
+        assert json.loads(result) == [["Complex ERROR: Only IfcPhysicalSimpleQuantity is supported", 0.0, ""]]