MySql function tax (calculate IVA)

Determine what datatypes you want. INT won't work so hot with your 1.16 idea. You lose to rounding to an INT. The below does not use INT

DROP FUNCTION IF EXISTS CalculoIVA;
DELIMITER $$
CREATE FUNCTION CalculoIVA
(x DECIMAL(12,4))
RETURNS DECIMAL(12,4) 
DETERMINISTIC 
BEGIN 
  DECLARE IVA DECIMAL(12,4); -- OR FLOAT etc
  SET IVA = x *1.16; 
  RETURN IVA;
END;$$
DELIMITER ;

select CalculoIVA(9.12);
-- 10.5792

If you want the datatypes to be DECIMAL(12,2) or FLOAT just make the necessary changes.

Note: the DELIMITER wrapper is unnecessary for PHPMyAdmin users.