The water-cement ratio, also known as water/cement ratio, w/c, or cementitious paste is one of the most important parameters of concrete technology, as it considerably influences its final strength.

It expresses the intimate relationship that exists between the weight of the water used in the mixture and the weight of the cement. Since it is mathematically a ratio, a division sign (slash: / ) should be used and never a hyphen.

Since the weight of the water used must always be less than the weight of the cement, the resulting figure is less than unity.

A low water/cement ratio leads to a higher strength concrete than a high water/cement ratio. But the higher this ratio is, the more workable the concrete will be.

The lowest w/c ratio to obtain complete hydration of the cement is considered equal to 0.42 and the highest 0.60.

The reason why the RAC should be between 0.42 and 0.60 (usually: one weight amount of water with respect to two weight amounts of cement with a variation of 25% excess or 18% under cement) is that the cement particles must be properly hydrated. An excess of water causes the particles to reach a point where they no longer absorb water, leaving empty spaces in which nothing acts.

The lack of water does not allow the particles to be sufficiently hydrated, so it does not fulfill the agglomerating function.