Utilitarianism is a prominent approach to social justice that has played a central role in economic theory. A key issue for utilitarianism is to define how utilities should be measured and compared. This paper draws on Harsanyi's approach (Harsanyi, 1955) to derive utilities from choices in risky situations. We introduce a new normalization of utilities that ensures that: 1) a transfer from a rich to a poor is welfare enhancing, and 2) populations with more risk averse people have lower welfare. We propose normative principles that reflect these fairness requirements and characterize fair utilitarianism. We also study some implications of fair utilitarianism for risk sharing and collective risk aversion.