A novel method was proposed for calculating the flux of an ultrafiltration membrane in water treatment processes, and all necessary parameters for this simulation were determined solely from membrane sheet tests. By comparing the water flux of the commercial element with the data from the membrane sheet measurement, this investigation found that the tendency of water flux variation with time in the membrane element was similar to that which occurred in the membrane sheet. Therefore, it is possible to express water flux variation of the membrane element based on the results from the membrane sheet measurement using a practical water source. Wastewater from the discharge of an activated sludge process was employed to carry out a pilot test with an 8-inch spiral element, made of polyvinylidene fluoride (PVDF) with a molecular weight cut-off of 150 kDa, and water fluxes under various transmembrane pressures were collected. Calculations were approximately consistent with that of the pilot test, which enables us to believe that the proposed method is reliable for designing a practical ultrafiltration system.