diff --git a/backend/tests/user/api/test_user_viewsets.py b/backend/tests/user/api/test_user_viewsets.py
index 807aa08..d88a19a 100644
--- a/backend/tests/user/api/test_user_viewsets.py
+++ b/backend/tests/user/api/test_user_viewsets.py
@@ -118,7 +118,7 @@ def test_update_persists_color_palette(self, api_client, user_factory):
 
     @pytest.mark.parametrize(
         'payload',
-        [{'planOverrides': None}, {'layoutNavigationStyle': 'x'}, {'colorPalette': 'x'}],
+        [{'planOverrides': None}, {'layoutNavigationStyle': 'x'}, {'colorPalette': 'x'}, {'supplyCartDays': -1}],
     )
     def test_update_rejects_invalid_values(self, api_client, user_factory, payload):
         user = user_factory(id=1)
@@ -142,6 +142,26 @@ def test_retrieve_repairs_invalid_stored_values(self, api_client, user_factory):
         assert response.data['layoutNavigationStyle'] == 'full'
         assert response.data['colorPalette'] == 'default'
 
+    def test_update_accepts_decimal_supply_cart_days(self, api_client, user_factory):
+        user = user_factory(id=1)
+
+        response = api_client.as_user(user).patch(
+            reverse('user:user_preferences'), data={'supplyCartDays': 2.5}, format='json'
+        )
+
+        assert response.status_code == 200
+        assert response.data['supplyCartDays'] == 2.5
+        assert UserPreference.objects.get(user=user).preferences['supply_cart_days'] == 2.5
+
+    def test_retrieve_reads_stored_integer_supply_cart_days(self, api_client, user_factory):
+        user = user_factory(id=1)
+        baker.make(UserPreference, user=user, preferences={'supply_cart_days': 7})
+
+        response = api_client.as_user(user).get(reverse('user:user_preferences'))
+
+        assert response.status_code == 200
+        assert response.data['supplyCartDays'] == 7
+
     def test_partial_update_keeps_omitted_keys(self, api_client, user_factory):
         user = user_factory(id=1)
         stored = {
diff --git a/backend/user/api/serializer.py b/backend/user/api/serializer.py
index 8c125fb..39cb48f 100644
--- a/backend/user/api/serializer.py
+++ b/backend/user/api/serializer.py
@@ -178,7 +178,7 @@ class UserPreferenceSerializer(JSONSafeSerializerMixin, serializers.Serializer):
     burnDaysYellow = serializers.IntegerField(source='burn_days_yellow', min_value=0, default=10, required=False)
     burnResupplyDays = serializers.IntegerField(source='burn_resupply_days', min_value=0, default=18, required=False)
     burnOrigin = serializers.CharField(source='burn_origin', required=False)
-    supplyCartDays = serializers.IntegerField(source='supply_cart_days', min_value=0, default=20, required=False)
+    supplyCartDays = serializers.FloatField(source='supply_cart_days', min_value=0, default=20, required=False)
     layoutNavigationStyle = serializers.ChoiceField(
         source='layout_navigation_style', choices=LAYOUT_NAVIGATION_STYLES, required=False
     )