This paper offers the model that can estimate the cementing efficiency of fly ash (k value) based on a mix proportion of concrete containing fly ash (FA). The prediction model was derived using various statistical analyses, based on a wide range of mix proportions and a number of strength test results of ready mixed concretes used in eight construction sites. The k value increases with increasing water-binder ratio. As the FA replacement ratios increase, the k value increases at FA replacement ratios of less than 15%, but decreases at ratios of 15% or more. The k values obtained from the cementing efficiency estimate model range from 0.1 to 2.1.