["Sociology of Health & Illness, Volume 43, Issue 6, Page 1437-1453, July 2021. ", "\nAbstract\nDespite the ubiquity of the body mass index (BMI) in discourse on health, there is ambiguity in its use as a biomarker of current abnormality versus future risk. This distinction is consequential for knowledge of the relationship between body size and health, as well as for individuals deemed to have abnormal and ‘unhealthy’ bodies. Consequently, the purposes of this review are threefold. The first is to differentiate this ‘biomarker’ perspective from extant critiques of BMI as a proxy for health behaviours or as the defining characteristic of obesity as a disease. The second is to highlight the shift towards treating BMI as a measure of attained unhealthiness, rather than a probabilistic indicator of risk. Finally, rather than call for the abolition of BMI, this paper argues that its continued use as ‘just a number’ is in keeping with the push for weight neutrality in research and practice. The review concludes by demonstrating how the riskiness and unhealthiness of body size is conflated in public health messaging on COVID‐19. BMI is a marker of risk, but its use as a surrogate for COVID‐19 severity equates body size with health, shaping beliefs about vulnerability and personal responsibility amid an ongoing pandemic.\n"]