Many welfarists wish to assign to each possible state of the world a numerical value that measures something like its moral goodness. How are we to determine this quantity? This paper proposes a contractualist approach: a legitimate measure of moral goodness is one that could be justified to each member of the population in question. How do we justify a measure of moral goodness to each individual? Each individual recognises the measure of moral goodness must be a compromise between the (...) different levels of well-being within the population. Some compromises are more reasonable than others; and some are better justifiable to a given member of the population than others. Each member recognises that the social chooser's measure of moral goodness is going to have to deviate from the well-being function of at least some of the members of the population. But we can nonetheless justify it to each of them if it doesn't deviate more than is necessary, and if the deviations from each member are given equal weighting in whatever process we use to determine it. This paper proposes that we begin with a measure of the distance from a proposed compromise to an individual's level of well-being, and then say that the moral goodness is the candidate compromise that minimizes the sum of distances from it to the individuals' levels of well-being. I describe a range of such measures of distance and show that some give utilitarianism, others different versions of prioritarianism. (shrink)