A system, device and method of determining the 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 a first heart rate for the person during the first predetermined time period； subsequent to determining that the first posture of the person satisfies first posture criteria for a first predetermined time period, determining that a second posture of the person satisfies second posture criteria for at least a second predetermined period； determining a second heart rate for the person during the second predetermined time period； determining a change in heart rate as the second heart rate minus the first average 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.
