I don't agree with that. Utilitarian ethics don't specify how the utility function is calculated, especially how you make the aggregate function from all the individual utility.

I was referring to total and average utilitarianism, the two most common kinds.

I fear that, like the human utility function is complicated, the aggregate function we should use is complicated and will contain sum, average, median and gini in a form or in another.

I agree completely. I think we'll probably have to use multiple methods of aggregation and then combine the score in some way.