Water Cement Ratio Calculator

In short A water/cement ratio is TOTAL water divided by the weight of cement, and the word total is where batches go wrong: damp sand and damp aggregate already carry water, and that water counts whether anybody measures it or not. One 50 kg bag of cement at a ratio of 0.50 wants 25 litres of water in the mix. If the batch also has 100 kg of sand at 4% moisture and 200 kg of aggregate at 1%, those bring 3.8 and 2.0 litres with them — 5.8 litres already present — so only 19.2 should be added at the mixer. Pour in the full 25 on top of wet sand and the real ratio is 0.62, not 0.50. THE IDENTITY IS WHAT MAKES THIS WORTH DOING PROPERLY: water added plus water already there, divided by cement, equals the target ratio exactly, at any moisture content whatsoever. Change how wet the sand is and the amount you add changes; the ratio does not move. That is the whole point of accounting for free water rather than eyeballing it. There is a second trap in the moisture figure itself. Moisture is quoted as a percentage of the DRY weight, so 100 kg of sand at 4% is not 4 kg of water — it is 100 divided by 1.04, giving 96.2 kg of dry sand carrying 3.8 kg of water. The difference is small at 4% and grows quickly as the sand gets wetter. This page also shows what one more bucket does: ten extra litres on that batch takes the ratio from 0.50 to 0.70, a rise of 40%. AND HERE IS WHAT THIS PAGE DOES NOT DO. It does not tell you which ratio to use, what strength you will get, or whether your mix will work. It is exact arithmetic on the numbers you enter, and every one of those numbers comes from your specification rather than from this site.