Concrete Mix Ratio Calculator
Works out the cement, sand, aggregate and water for a batch of concrete at standard mix ratios. Useful when you are batching on site rather than buying pre-mixed bags.
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The formula
dry volume = wet volume × 1.54, split by the ratio parts, then × bulk density
Dry materials take up about 54% more space than the concrete they make, because the cement and sand fill the gaps between the stones. Skipping that factor is why hand-batched mixes come up short.
What to watch out for
Two things decide whether a site-batched mix works:
- The 1.54 dry factor. A cubic metre of finished concrete needs about 1.54 m³ of loose dry material, because the fine material fills the voids in the coarse. Batching to the wet volume leaves you roughly a third short.
- The water ratio decides the strength. More water makes concrete easier to place and permanently weaker. 0.5 by weight is the usual target; every 0.05 above that costs meaningful strength.
Ratios are by volume, which is how they are batched on site — a bucket of cement to two of sand to four of stone. Batching by weight is more accurate but rarely practical outside a plant.
Frequently Asked Questions
What is the standard concrete mix ratio?
1:2:4 (cement:sand:aggregate) is the general-purpose mix, giving roughly C20 strength. 1:1.5:3 gives about C25 for slabs and footings; 1:1:2 gives about C30 for structural work.
How much cement for 1 m³ of concrete?
At a 1:2:4 mix, about 320 kg — roughly 13 × 25 kg bags or 7 × 50 kg bags. At 1:1.5:3 it rises to about 395 kg.
Why multiply by 1.54?
Because dry aggregate has air gaps that the sand and cement fill during mixing. The loose materials occupy about 54% more volume than the concrete they produce.
What water-cement ratio should I use?
0.45 to 0.55 by weight. Lower is stronger but harder to place; above 0.6 the strength falls away quickly. Wet-looking concrete is weak concrete.