Confusingly, ‘utility’ in this case does not refer to usefulness, but to a sort of combo of happiness and wellbeing. When a utilitarian tries to decide how to act, they take stock of all the probable outcomes, and what sort of ‘utility’ or happiness will be brought about for all parties involved. This process is sometimes referred to by philosophers as ‘utility calculus’. When I am trying to calculate the expected net utility gain from a projected set of actions, I am engaging in ‘utility calculus’ (or, in normal words, utility calculations).
Utility should be calculated when we consider what the behavior or goods would bring to us. In economic sense, people will choose the one with the maximum utility. Also in philosophy, net utility is widely considered.