The ponderal index is a measure of body leanness or corpulence calculated as weight divided by the cube of height, essentially the inverse of the better-known body mass index relationship. Expressed as kilograms per cubic metre, it was proposed as an alternative to BMI that scales more correctly with body proportions, since weight scales with volume (the cube of height) rather than area. It finds particular use in neonatal assessment, where the ponderal index distinguishes symmetric from asymmetric growth restriction in newborns, and in some epidemiological research. Like BMI, it cannot distinguish fat from muscle and is an imperfect proxy for adiposity at the individual level. The index’s persistence in obstetric and paediatric literature shows the long search for a simple anthropometric number that captures what scales and tape measures cannot directly see.