A system, device and method of determining a probability of dehydration of a person is provided. In one embodiment, the method comprises receiving data of a heart rate of the person； receiving data of a posture of the person； determining that a first posture of the person satisfies first posture criteria for a first predetermined time period； determining that the posture of the person satisfies a similarity threshold with a posture envelope； determining a first heart rate for the person while in a first posture； determining a second heart rate for the person while in the second posture； determining a change in heart rate as a difference between the second heart rate and the first heart rate； determining a first probability of dehydration based at least in part on the change in heart rate； and outputting the first probability of dehydration.
