Various implementations disclosed herein include devices, systems, and methods that determines a distance between a portion (e.g., top) of a person's head and the floor below as an estimate of the person's height. For example, an example process may include determining a first position on a head of a person in a three-dimensional (3D) coordinate system, the first position determined based on detecting a feature on the head based on a two-dimensional (2D) image of the person in a physical environment, determining a second position on a floor below the first position in the 3D coordinate system, and estimating a height based on determining a distance between the first position and the second position.