Both carotid artery SBP and central SBP obtained via a transfer function are highly sensitive to the calibration of the respective carotid artery and radial artery pressure waveforms. Our data suggest that the one-third rule to calculate MAP from brachial cuff BP should be avoided, especially when used to calibrate radial artery pressure waveforms for subsequent application of a pressure transfer function. Until more precise estimation methods become available, it is advisable to use 40% of brachial pulse pressure instead of 33% to assess MAP.