The technology described in this document can be embodied in a method that includes obtaining a first pulse transit time (PTT) measurement and a second PTT measurement for a subject. The PTT represents a time taken by a pulse pressure wave traveling to the wrist of the subject. The first PTT is measured when the wrist of the subject is at a first height, and the second PTT is measured when the wrist of the subject is at a second height. The method also includes computing a hydrostatic difference in blood pressure as a function of the difference between the first height and the second height, and computing a calibration metric as a function of (i) the hydrostatic difference in blood pressure, and (ii) the difference between the first PTT and the second PTT.
