Cement Mix Calculator
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.
How to use the cement mix calculator
- Enter the desired finished (compacted) volume of the mix, in cubic meters.
- Enter the mix ratio between cement, sand and gravel — 1:2:3 is a common standard mix for general construction concrete.
- The calculator shows how many kg of each ingredient you need, plus the number of 25 kg bags of cement.
How the mix is calculated
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.
The water-cement ratio decides the strength
It is tempting to add plenty of water, because the mix becomes easier to work. It is also the most common way to ruin the strength. Excess water that does not take part in the chemical curing evaporates afterwards and leaves pores behind, and porous concrete is weaker and less frost-resistant. A low water-cement ratio gives stronger concrete — if the mix is too stiff to work with, admixtures are a better answer than more water.
Curing is not the same as drying, either. Concrete gains strength through a chemical reaction that requires moisture, and a surface that dries out too quickly in sun or wind cures poorly and can develop shrinkage cracks. Fresh concrete is therefore kept damp or covered for the first few days, and casting in cold conditions requires its own measures because the reaction proceeds very slowly near freezing.
Frequently asked questions
Why is the dry volume larger than the finished volume?
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.
What mix ratio should I use?
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.
Should I buy ready-mix concrete instead?
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.