<p>Research on children’s well-being in joint physical custody (JPC), where children alternate between their parents’ homes after family dissolution, is growing. However, researchers operationalize JPC using different measures, such as a residential calendar, the number of children’s overnights, or a general self-assessment. Furthermore, they employ different thresholds ranging from 25% to 50% to define JPC. A discussion of the differences between definitions and measures, as well as an empirical test of the implications, does not exist. Therefore, this study investigates whether the type of measurement of post-separation care arrangements affects their classification as sole physical custody (SPC) or joint physical custody (JPC) and, consequently, the results regarding children’s well-being in these families. Analyses are based on the Family Models in Germany (FAMOD) study, which encompasses various measures of post-separation care arrangements. Data were collected in 2019, and the analytical sample comprised 1,104 post-separation families. Results revealed that different measurements and operationalizations of physical custody arrangements affect the number of families classified as JPC and SPC families, but do not affect the results of the children’s mental health (SDQ) used as an example of children’s well-being. Future research should reflect on the measurement of post-separation care arrangements, as they have an impact on the quantity of SPC and JPC families and, more importantly, differ qualitatively, e.g., when operationalized in terms of days or nights. However, using different measures is not problematic for analyses of the effects of physical custody arrangements, as the results are consistent across them.</p>