Abstract: In this paper, the existing problems of the current scalability models are analyzed, and aiming at the requirements of the practical evaluations and tests, a practical scalability metric based on iso-average-computation-load is proposed to provide a quantitative measurement of the scalability. In this metric, the definitions of scalable speedup and scalability are different from the current metrics. By using this metric, a practical method can be obtained to test scalable speedup and scalability, and combined with curve fitting or parallel computing time model, the scalabilities of parallel systems can be predicted.