diff --git a/purchase_only_by_packaging/models/purchase_order_line.py b/purchase_only_by_packaging/models/purchase_order_line.py
index 115d8e207cd..aaf24a9d256 100644
--- a/purchase_only_by_packaging/models/purchase_order_line.py
+++ b/purchase_only_by_packaging/models/purchase_order_line.py
@@ -50,3 +50,13 @@ def _force_qty_with_package(self):
     @api.onchange("product_qty")
     def _onchange_product_qty(self):
         self._force_qty_with_package()
+
+    @api.depends("product_packaging_id")
+    def _compute_product_qty(self):
+        res = super()._compute_product_qty()
+        for item in self.filtered(lambda x: x.product_packaging_id):
+            old_qty = item.product_qty
+            item.product_qty = item.product_id._convert_packaging_qty(
+                old_qty, item.product_uom, packaging=item.product_packaging_id
+            )
+        return res
diff --git a/purchase_only_by_packaging/tests/test_purchase_only_by_packaging.py b/purchase_only_by_packaging/tests/test_purchase_only_by_packaging.py
index fb7092c7924..b11ac076807 100644
--- a/purchase_only_by_packaging/tests/test_purchase_only_by_packaging.py
+++ b/purchase_only_by_packaging/tests/test_purchase_only_by_packaging.py
@@ -49,11 +49,11 @@ def test_convert_packaging_qty(self):
         """
         self.product.purchase_only_by_packaging = True
         packaging = self.packaging_tu
-        self.order_line.product_packaging_id = packaging
         # For this step, the qty is not forced on the packaging
         # But the warning will be raise because the value of packaging qty is
         # not integer package
         with self.assertRaises(ValidationError):
+            self.order_line.product_packaging_id = packaging
             self.order_line.product_packaging_qty = 0.6
             self.assertAlmostEqual(
                 self.order_line.product_qty, 12, places=self.precision