A trend of mixing proportion design of fly ash concrete is to set up the direct method that does not need reference mixing proportion. To achieve this, the strength formula must be set up. The paper describes the methods of setting up the two variable strength formula by using computer and the method of improving the accuracy and universality of the strength formula through the optimal procedure. Comparison between the result of calculated strength and determined strength shows that the two variable strength formula with the optimal procedure has good universality.
