If not estimated by a measuring device, the mean arterial pressure (MAP) can be approximated from the measured systolic (SBP) and diastolic blood pressure (DBP) by applying a traditional formula MAP=SBP+k.(SBP-DBP), where the pulse pressure coefficient k is equal to 0.33 in most common implications. Our aim was to study whether this value can validly be applied to calculate MAP of critically ill patients. A total of 19 patients after cardiac surgery with an intra-radial cannula were involved. SBP, DBP and MAP were obtained from the recorded arterial pressure waveforms. The value for k was found to be 0.31±0.04. Thus, MAP values obtained by applying the traditional one-third approximation were overestimated by 1.7±2.3 mmHg (p<0.001).