In this study, a biofouling index based on the relative pressure drop is presented to quantitatively evaluate the amount of fouling in spacer-filled membrane filtration channels. The biofouling index was defined as the inverse of the time to reach a relative pressure drop of 100% and can be interpreted as a fouling rate or cleaning frequency. The index was applied to evaluate biofilm growth in membrane fouling simulators with reverse osmosis membranes and commercial feed spacers operated with different feed water nutrient concentrations and crossflow velocities. Biofilm accumulation on the membrane and feed spacer was characterized in situ using optical coherence tomography. We showed that the biofouling index is directly related to the volume of biofouling independent of the applied crossflow velocity and a suitable tool for improved quantitative comparison of the biofouling rate. Furthermore our results suggest that the pressure drop is better described as function of the velocity at the perimeter of a spacer cell instead of the average velocity in the channel. Although the biofouling index is developed for biofouling, the index may be applied to quantitatively assess mitigation strategies in spacer filled channels for a wider range of fouling types.