The present invention relates to an apparatus and method for determining blood pressure of a subject. To automatically trigger calibration the apparatus comprises a sensor signal input configured to obtain an arterial pulse wave sensor signal of the subject, a feature extraction unit configured to extract multiple features from the obtained arterial pulse wave sensor signal, an estimation unit configured to determine multiple blood pressure estimation values for individual extracted features and/or groups of extracted features and to determine the subject's blood pressure from said multiple blood pressure estimation values, a calibration unit configured to calibrate the estimation unit based on blood pressure reference measurements, and a calibration trigger unit configured to trigger calibration by the calibration unit if the multiple blood pressure estimation values diverge more than a divergence limit.