Research efforts over the past many decades towards an effective utilisation of fly ash in concrete do not seem to have led to a quantitative understanding of the efficiency of fly ash as a mineral admixture, particularly because of the vast variation in the grades of concrete and the different levels of replacements adopted. This paper is an effort directed towards a specific understanding of the efficiency of fly ash in concrete, considering the strength to water cement ratio relations, age and percentage of replacement The "overall cementing efficiency" (k) of fly ash was established through a "general efficiency factor" (ke), dependent on the age and a "percentage efficiency factor" (kp), dependent on the replacement percentage. This information will allow designers to evaluate the strength of well designed fly ash concretes at any age and at the different percentages of replacement.