These calculators are used at your own risk. The formulas used for the calculations are a combination of our own experience as well as published recommendations by different cement manufacturers. No waste is allowed for. The rounding off, of quantities within the formulas will give slight differences in strength. We do not guarantee the calculations. These quantities are indicative only and should not be used where mortar/concrete/ etc. is specified by strength or class. Users should also note that there are many other factors that influence the strength of mortar and concrete respectively, like the quality of sand/water/cement will have a influence on strength and workability, these factors should be considered when ordering materials. Should a user pickup discrepancies in our formulas or assumptions, please notify us thereof, we always strive to make things better