Hydration characteristics as well as the associated strength-durability properties of a slag modified concrete differ largely with that of an ordinary cement concrete. The properties of the supplementary additives have their influences on the behaviour of the concrete, both in fresh and hardened state. Strength-cementitious material-water relationship for Portland Slag Cement (PSC) concrete incorporating the characteristics of slag and their effects on hydration mechanism is expected to be a convenient tool for proportioning such blended concrete. This paper deals with the development and suitability study of such relationship on the basis of experimental findings for proportioning slag based concrete involving strength ratio or the ratio of compressive strength of concrete mix to that of the standard mortar mixture and water-cementitious material ratio .