diff --git a/addons/mrp/models/mrp_production.py b/addons/mrp/models/mrp_production.py
index e7547f22e34..1e29203d220 100644
--- a/addons/mrp/models/mrp_production.py
+++ b/addons/mrp/models/mrp_production.py
@@ -625,7 +625,7 @@ class MrpProduction(models.Model):
                 move[0].with_context(do_not_unreserve=True).write({'product_uom_qty': quantity})
                 move[0]._recompute_state()
                 move[0]._action_assign()
-                move[0].unit_factor = remaining_qty and quantity / remaining_qty or 1.0
+                move[0].unit_factor = remaining_qty and (quantity - move[0].quantity_done) / remaining_qty or 1.0
                 return move[0], old_qty, quantity
             else:
                 if move[0].quantity_done > 0:
diff --git a/addons/mrp/tests/test_order.py b/addons/mrp/tests/test_order.py
index a6bc2d47566..f13f41d44ae 100644
--- a/addons/mrp/tests/test_order.py
+++ b/addons/mrp/tests/test_order.py
@@ -363,6 +363,42 @@ class TestMrpOrder(TestMrpCommon):
         self.assertEqual(sum(mo.move_raw_ids.filtered(lambda m: m.product_id == p1).mapped('quantity_done')), 20)
         self.assertEqual(sum(mo.move_finished_ids.mapped('quantity_done')), 5)
 
+    def test_update_quantity_3(self):
+        """ Build 1 final products then update the Manufacturing
+        order quantity. Check the remaining quantity to produce
+        take care of the first quantity produced."""
+        self.stock_location = self.env.ref('stock.stock_location_stock')
+        mo, bom, p_final, p1, p2 = self.generate_mo(qty_final=2)
+        self.assertEqual(len(mo), 1, 'MO should have been created')
+
+        self.env['stock.quant']._update_available_quantity(p1, self.stock_location, 20)
+        self.env['stock.quant']._update_available_quantity(p2, self.stock_location, 5)
+        mo.action_assign()
+
+        produce_form = Form(self.env['mrp.product.produce'].with_context({
+            'active_id': mo.id,
+            'active_ids': [mo.id],
+        }))
+        produce_form.qty_producing = 1
+        produce_wizard = produce_form.save()
+        produce_wizard.do_produce()
+
+        update_quantity_wizard = self.env['change.production.qty'].create({
+            'mo_id': mo.id,
+            'product_qty': 3,
+        })
+        update_quantity_wizard.change_prod_qty()
+
+        produce_form = Form(self.env['mrp.product.produce'].with_context({
+            'active_id': mo.id,
+            'active_ids': [mo.id],
+        }))
+        produce_wizard = produce_form.save()
+        produce_wizard.do_produce()
+        mo.button_mark_done()
+        self.assertEqual(sum(mo.move_raw_ids.filtered(lambda m: m.product_id == p1).mapped('quantity_done')), 12)
+        self.assertEqual(sum(mo.move_finished_ids.mapped('quantity_done')), 3)
+
     def test_rounding(self):
         """ Checks we round up when bringing goods to produce and round half-up when producing.
         This implementation allows to implement an efficiency notion (see rev 347f140fe63612ee05e).