diff --git a/addons/account/models/account.py b/addons/account/models/account.py
index d0aca4fafe4..ff6ee0bb1c7 100644
--- a/addons/account/models/account.py
+++ b/addons/account/models/account.py
@@ -613,7 +613,7 @@ class AccountTax(models.Model):
         """
         self.ensure_one()
         if self.amount_type == 'fixed':
-            return math.copysign(self.amount, base_amount) * quantity
+            return math.copysign(self.amount, base_amount) * abs(quantity)
         if (self.amount_type == 'percent' and not self.price_include) or (self.amount_type == 'division' and self.price_include):
             return base_amount * self.amount / 100
         if self.amount_type == 'percent' and self.price_include: