A blood pressure calculating device operable to be worn on a user's limb, the blood pressure calculating device comprising: a sensor system (210, 220) operable to observe a frequency shifted signal associated with back scattering of the user's blood, the sensor system comprising an ultrasonic sensor and an elevation sensor; and a processor, the blood pressure calculating device being characterised by the processor being operable to: process the frequency shifted signal from the sensor system to determine values of arterial lumen and values of blood flow during measurements of two different elevations of the user's limb; determine a first coefficient of an arterial stress-strain relationship based on the determined values of arterial lumen at the two different elevations of the user's limb; determine a hydrostatic pressure difference between the two elevations based on a distance between the two elevations provided by the elevation sensor; calculate a second coefficient of the arterial stress-strain relationship based on the determined values of the arterial lumen and blood flow, the hydrostatic pressure difference between the different elevations of the user's limb, and the first coefficient; and calculate a blood pressure based on the arterial stress-strain relationship using the first coefficient, the second coefficient and the values of arterial lumen. a sensor system (210, 220) operable to observe a frequency shifted signal associated with back scattering of the user's blood, the sensor system comprising an ultrasonic sensor and an elevation sensor; and a processor, the blood pressure calculating device being characterised by the processor being operable to: process the frequency shifted signal from the sensor system to determine values of arterial lumen and values of blood flow during measurements of two different elevations of the user's limb; determine a first coefficient of an arterial stress-strain relationship based on the determined values of arterial lumen at the two different elevations of the user's limb; determine a hydrostatic pressure difference between the two elevations based on a distance between the two elevations provided by the elevation sensor; calculate a second coefficient of the arterial stress-strain relationship based on the determined values of the arterial lumen and blood flow, the hydrostatic pressure difference between the different elevations of the user's limb, and the first coefficient; and calculate a blood pressure based on the arterial stress-strain relationship using the first coefficient, the second coefficient and the values of arterial lumen. process the frequency shifted signal from the sensor system to determine values of arterial lumen and values of blood flow during measurements of two different elevations of the user's limb; determine a first coefficient of an arterial stress-strain relationship based on the determined values of arterial lumen at the two different elevations of the user's limb; determine a hydrostatic pressure difference between the two elevations based on a distance between the two elevations provided by the elevation sensor; calculate a second coefficient of the arterial stress-strain relationship based on the determined values of the arterial lumen and blood flow, the hydrostatic pressure difference between the different elevations of the user's limb, and the first coefficient; and calculate a blood pressure based on the arterial stress-strain relationship using the first coefficient, the second coefficient and the values of arterial lumen.