The dairy sector is facing difficulties in its environmental sustainability. A method is presented to assess sustainability based on the analysis of energy flows, with a particular focus on circularity. The results show that intensive dairy production systems, even those based on grazing, present an unbalanced situation in flow circularity and mobilizing more resources than they provide. This type of analysis is a first step in the research of sustainable solutions for the dairy sector, and where the measurement of carbon and nitrogen flows would be complementary.