Rather than critically examining the uses of self-trackers—digital personal quantification tools—within the “quantified self” movement that emerged in California, this article studies their coherence as an extension of ordinary self-care techniques. It analyzes concrete practices of personal quantification in relation to stages in the life cycle by combining a questionnaire survey (n=1,829) with a large qualitative survey (n=105). The questionnaire survey reflects the significant place occupied by figures in self-care. It analyzes the practices of respondents, 28% to 43% of whom (depending on their age group) engage in digital self-tracking practices, and 14% to 27% of whom own a connected measuring device. The qualitative part of the study examines the intertwining issues at the heart of these practices and shows that, despite the diversity of individual contexts, the goals of self-quantification evolve according to the individual’s age and stage in the life cycle. While regulating an unstable lifestyle through self-measurement is a widespread objective among younger people, the need to rationalize professional, domestic, and personal lives often becomes central in the uses of quantification following the birth of children. After the age of fifty, it gives way to a concern with preventing threats related to aging.