For efficient mix design, this paper offers following information: the fly ash (FA) replacement ratio in which strength development is the maximum, the range of FA replacement ratio in which the strength of FA concrete is higher than that of ordinary Portland cement (OPC) concrete, and the binder content per unit strength. For these matters, various statistical analyses were performed 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 strength ratio, the ratio of FA concrete to OPC concrete in compressive strength, was expressed as the function of binder-water ratio and FA replacement ratio by multiple linear regression analysis. This equation suggests the range of FA replacement ratios in which strength ratio of 1 or higher can be obtained when binder content of FA concrete is the same as that of OPC concrete. The strength ratio increases with increasing water-binder ratio. As water-binder ratios decrease, the range of FA replacement ratios also decreases. It is desirable that FA concrete should have FA replacement ratio of about 15% to guarantee required average compressive strength using minimum binder contents. In FA replacement ratio of about 15%, the binder content per unit strength is 10.1~10.6kg/$m^3$/MPa with respect to water-binder ratio. Because the binder content per unit strength is calculated with respect to water-binder ratio and FA replacement, its concept can be used as a means of determination of the binder content in mix design of FA concrete.