Normative theory that human conduct is right or wrong because of its tendency to produce favorable or unfavorable consequences for the people who are affected by it. The hedonistic utilitarianism of Bentham, Mill, and Sidgwick maintains that all moral judgments can be derived from the greatest happiness principle. The ideal utilitarianism espoused by G. E. Moore, on the other hand, regarded aesthetic enjoyment and friendship as the highest ethical values. Contemporary utilitarians differ about whether the theory should be applied primarily to acts or rules.
(Via Philosophical Dictionary)
It strikes me that utilitarianism is only half of a moral theory. In order to determine what counts as a favorable or unfavorable consequence, it seems to me you need another theory of what’s actually right and wrong. Especially, in this context, a theory of what is good for a human being, what makes a human being flourish – what the proper telos is for a person (or group or society).
Without knowing that, how could you begin to decide what counts as a favorable or unfavorable consequence for an individual person? Only if you know what’s good for a human being, can you decide what will work to that human being’s favor. So, it seems we need some moral guidelines before we can calculate the possible consequences of actions.