diff --git a/addons/product/product.py b/addons/product/product.py
index db329313428..a085e4d15ed 100644
--- a/addons/product/product.py
+++ b/addons/product/product.py
@@ -122,6 +122,7 @@ class product_uom(osv.osv):
 
     _sql_constraints = [
         ('factor_gt_zero', 'CHECK (factor!=0)', 'The conversion ratio for a unit of measure cannot be 0!'),
+        ('factor_category_id_uniq', 'unique (category_id, factor)', 'You can not have more than one UOM with same factor for same UOM category'),
     ]
 
     def _compute_qty(self, cr, uid, from_uom_id, qty, to_uom_id=False):