Estimating the fouling potential is a prerequisite for controlling membrane fouling in reverse osmosis systems. Two different tests are routinely used to that end: the Silt Density Index (SDI) and the Modified Fouling Index (MFI0.45). SDI test, however, has disadvantages which make it unreliable. One disadvantage is that no linear relationship exists between SDI and the colloidal concentration in the water. Besides that, the SDI is not based on a filtration model. Finally, the SDI is not corrected for the testing condition parameters such as temperature, pressure and membrane properties. The SDI test is simple and does not require advanced professional skills, but the above mentioned disadvantages make it unreliable. This work offers practical tools to resolve these disadvantages, such as a slide tool to calculate the SDI from the measured collection times. It also proposes a normalized SDI (SDI+), and offers a line chart and slide wheel charts for this conversion, thus correcting for different test conditions and membrane resistance. Finally, this paper introduces a new fouling index. This volume-based SDI, SDI_v, compares the initial flow rate with the flow rate after filtering the standard volume. It has a linear relationship to the particle concentration if complete blocking is the dominant fouling mechanism during the test, and is independent of testing conditions and membrane resistance. The mathematical model and experimental results show that SDI_v eliminates most of the disadvantages of the traditional SDI. The SDI_v is the second fouling index developed at the University of Twente, 30 years after the MFI0.45. (C) 2011 Elsevier B.V. All rights reserved.