This study examines the slimming practice produced by Internet-based weight-loss services and their use. Drawing on theories of practice, the study analyses the script of use that is constructed by the services, and the meanings, materialities and competences that are enacted in their use. Based on 20 semi-structured interviews with women who were users of two Finnish online weight-loss services, the study concludes that the services transform food into quantitative depictions of calories and nutrition. They configure slimmers as calculative agents and slimming as a practice based on incessant recording and monitoring. For online slimmers, the services acted in the double role of a control device with a focus on calorie restriction, and a learning device used to develop a skill of healthy eating. In the latter role, online slimming was hoped to result in an internalisation of a lifestyle change that would make calculation and constant monitoring unnecessary and the services redundant for their users. The results suggest that for its practitioners, online slimming is temporary rather than long-standing, but it may and is expected to act as a mediary in establishing other practices related to healthy lifestyles.