Work out how many kg of cement, sand and gravel you need for a self-mixed concrete or mortar batch, based on the desired volume and mix ratio.
Loose, dry materials (cement, sand, gravel) take up more space than the finished, compacted volume — a commonly used rule of thumb in concrete work is that the dry volume is about 1.54 times the finished volume. This dry volume is then split across the parts in the mix ratio, and each part is converted to kg using a typical bulk density:
1:2:3 (cement:sand:gravel) is a common mix for general construction purposes, while other ratios like 1:3:6 give a weaker mix suited to less load-bearing use. Check the recommendation for your specific project.
Because loose, dry materials have air gaps between the grains that disappear when mixed with water and compacted. This "shrinkage" is why you need to mix more loose material than the finished volume would suggest — typically figured at around 54% more.
1:2:3 (cement:sand:gravel) is a common standard mix for general construction concrete. For foundations and load-bearing structures, follow local building codes or a professional's recommendation, since the right ratio depends on load and use case.
For larger quantities, ready-mix concrete (delivered by truck) is often more cost-effective and gives more consistent quality than mixing it yourself. This calculator is most useful for smaller projects where you mix your own from bagged materials.