Migration-related detention – or the detention of non-citizens because of their status – is intimately associated with incarceration, raising questions about whether this form of detention is proportionate to the administrative aims of immigration policy. Many countries use prisons to hold irregular migrants during deportation proceedings; dedicated migrant detention facilities are frequently housed in abandoned jails; in some countries, purpose-built facilities are modelled on penitentiaries; and rights groups point to the “prison-like” qualities of detention centres to decry the perceived creeping criminalisation of immigration. This paper endeavours to use the legal principle of proportionality as a tool to critique immigration detention practices and policies. To this end, the article proposes a methodology for assessing operations at detention centres that opens the phenomenon up to empirical study and allows for comparative research of detention practices across a multiplicity of cases. It then highlights a discrete set of dimensions that can be used to measure the degree to which States’ employment of detention is proportional to the limited ends established in law for this type of deprivation of liberty.