While Foucault described population as the object of biopower he did not investigate the practices that make it possible to know population. Rather, he tended to overemphasise it as an object on which power can act. However, population is not an object awaiting discovery, but is represented and enacted by specific devices such as censuses and what I call population metrics. The latter enact populations by assembling different categories and measurements of subjects (biographical, biometric and transactional) in myriad ways to identify and measure the performance of populations. I account for both the object and subject by thinking about how devices consist of agencements, that is, specific arrangements of humans and technologies whose mediations and interactions not only enact populations but also produce subjects. I suggest that population metrics render subjects interpassive whereby other beings or objects take up the role and act in place of the subject.