Abstract Recent in-situ measurements show that major discrepancies exist between accurately measured net bit pressure drop values and those predicted by previously accepted models and equations. The direction of the discrepancies is such that the measured values of net bit pressure drop are 20 per cent lower than those predicted by the existing models. Subsequent laboratory data obtained under rigorously monitored and controlled conditions support the field-taken downhole data. These data, as well as a possible explanation, are presented in this paper.