Calculation of water flux in a spiral ultrafiltration element was conducted by the novel proposed method, in which all necessary parameters were determined solely from membrane sheet test. By comparing water flux of the commercial element with the data from membrane sheet measurement, this study found that the tendency of water flux variation with time in the membrane element was similar to that which occurred in the membrane sheet, including the consideration of scale-up effect due to hydrodynamics influence. Therefore, it is possible to express the variation of water flux in the membrane element based on the results from membrane sheet measurement using a practical water source. Surface water and sea water were separately employed to carry out a pilot test with an 8-inch spiral membrane element, made of polyvinylidene fluoride(PVDF) with a molecular weight cut-off of 150 kDa, and water fluxes under various transmembrane pressures. Calculations were approximate agreement with that of the pilot test, which enables us believe that the proposal method is reliable for designing a practical ultrafiltration system.