For batch mode ultra®ltration processes a numerical method for calculating membrane area has been devised with¯ux predicted from Flemmer's model. Erythromycin broth ®ltrate was used as a model¯uid. Experiments at a 20 dm 3 scale were used to estimate the parameters involved in Flemmer's equation, from which membrane areas appropriate for operations of 80 metric tons were calculated. Factors such as rejection, concentration ratio, etc, which affect the membrane size for batch operation, are discussed. The calculated results were consistent with experimental data at the 100 dm 3 scale. For continuous operation equations for calculating membrane areas have also been established. The minimum membrane area was calculated at the optimum concentration ratios of each stage, usually their values were different at each stages. Comparison between batch and continuous mode in the context of the membrane area required is also discussed. The method could be applicable to other ultra®ltration operations.