Find the cement, sand, aggregate and water a pour needs from its grade and size, in bags, cubic metres, cubic feet and kilograms.
Order 3 cubic metres of dry material for a 3 cubic metre slab and you will come up short by half. Sand and stone are full of air before the mixer turns, and that gap is where most concrete orders go wrong. This Concrete Mix Ratio Calculator applies the correction for you, then splits the result into cement, sand, aggregate and water.
Six nominal grades are built in, from lean M5 fill up to M25 for columns, each carrying the water-cement ratio normally used with it. A custom ratio box handles anything your specification asks for instead.
Load Sample opens on the commonest pour there is: M20 at 1:1.5:3, a slab 5 m by 4 m by 0.15 m, with 5% wastage allowed. That is 3 m³ of finished concrete, and it needs:
Drop the wastage to zero and the cement falls to 24.19 bags, which is the difference between ordering 26 bags and ordering 25 and hoping.
Wastage is added first, then the total is multiplied by the dry volume factor, 1.54 by default. So 3 m³ becomes 3.15 m³, then 4.851 m³ of loose material. That figure is divided in the mix proportions: with 1:1.5:3 the parts sum to 5.5, so cement takes one part and aggregate three. Cement converts to weight at 1,440 kg/m³ and then to bags, and the water follows from the cement weight rather than from the volume, because the water-cement ratio is a weight ratio. The factor itself is editable, since 1.52 and 1.57 are both defended in practice, but anything outside 1 to 2 is refused with “The dry volume factor is normally between 1.50 and 1.57 — values outside 1 to 2 are not realistic.”
Nominal mixes are proportioned by volume and are only sanctioned up to M25; above that a designed mix with tested materials is the right answer, which is why the grade buttons stop where they do. The bulk densities are design values and damp sand bulks up by a tenth or more, so a batch measured by box on a wet morning carries less sand than the arithmetic assumes. The least obvious control is the cement bag selector: switching from 50 kg to the US 94 lb bag changes nothing about the mix but rewrites the headline, because 1,270.08 kg is 25.40 bags one way and 29.79 the other.
Mix grade
Cement : sand : aggregate 1:1.5:3 · typical use: slabs, beams, domestic work · usual w/c 0.5
Work out the volume from
Cement needed
25.40 bags
1,270.08 kg of cement · 3 m³ of finished concrete · 4.851 m³ of dry material
| Material | m³ | cft | kg |
|---|---|---|---|
| Cement | 0.882 | 31.15 | 1,270 |
| Sand | 1.323 | 46.72 | 2,117 |
| Aggregate | 2.646 | 93.44 | 3,969 |
| Water | 635.04 litres | 635 | |
Cement, sand and aggregate by volume, in the ratio 1:1.5:3.
Dry materials total more than the wet pour, which is why 3.15 m³ of concrete needs 4.851 m³ of cement, sand and stone. The × 1.54 factor covers the air between the grains and the shrinkage once water is added.
Nominal mixes are proportioned by volume and suit work up to M25; anything stronger should come from a designed mix with tested materials. Bulk densities assume cement 1,440, sand 1,600 and aggregate 1,500 kg/m³, and damp sand bulks up by a tenth or more, so measure by weight where it matters.