Concrete volume and mix
Two questions in one: how much concrete the pour needs, and what goes into it. The volume is arithmetic with a waste allowance; the mix follows the ACI 211.1 absolute-volume method, which picks water from the slump, the water-cement ratio from the strength, and fills whatever is left with aggregate.
| Quantity | Value | Note |
|---|---|---|
| Shape | slab | 1 pour(s) |
| Volume per pour | 3.600 m³ | before waste |
| Waste allowance | 10% | spillage, over-excavation and finishing |
| Total volume | 3.960 m³ | 5.18 yd³ |
| Truck loads | 1 | at 9 m³ per truck |
| Formwork area | 3.00 m² | vertical faces |
| Target strength | 4000 psi | 28-day compressive |
| Maximum aggregate | 0.750 in | the largest nominal size |
| Slump | 3.0 in | workability |
| Air content | 2.0% | entrapped |
| Water | 345 lb/yd³ | 811 L total |
| Cement | 605 lb/yd³ | 359 kg/m³ |
| Fine aggregate | 1309 lb/yd³ | 777 kg/m³ |
| Coarse aggregate | 1674 lb/yd³ | 993 kg/m³ |
| Water-cement ratio | 0.570 | by weight |
| Batch weight | 3933 lb/yd³ | total per cubic yard |
| Proportions by weight | 1 : 2.16 : 2.77 : 0.57 | cement : fine : coarse : water |
| Cement for this pour | 1422 kg | 33.4 barrels of 42.6 kg |
| Water for this pour | 811 L | add less for aggregate moisture |
| Absolute volume | 27.00 ft³ | should close at 27.00 |
The batch weights assume oven-dry aggregates. Site aggregate carries moisture, so the added water has to be reduced by the free water in the sand and stone — the single most common reason a mix comes out too wet.
V = L × W × T × (1 + waste) · Plus the waste allowance for spillage and over-excavation.
w from slump and aggregate size · ACI 211.1 Table 6.3.3.
c = w / (w/c) · The ratio comes from the strength table.
27 = V_w + V_c + V_air + V_fine + V_coarse · Each component is its weight over its specific gravity times 62.4.
A clear path from inputs to results.
Two questions in one: how much concrete the pour needs, and what goes into it. The volume is arithmetic with a waste allowance; the mix follows the ACI 211.1 absolute-volume method, which picks water from the slump, the water-cement ratio from the strength, and fills whatever volume is left in a cubic yard with aggregate.
How to use
- Choose the shape and enter its dimensions.
- Set the number of pours and the waste allowance.
- Enter the target strength and the maximum aggregate size.
- Add the slump, the fineness modulus and whether the concrete is air-entrained.
- Read the volume, the batch weights and the proportions.
The formula
Reference: ACI 211.1, Standard Practice for Selecting Proportions for Normal, Heavyweight and Mass Concrete, Tables 6.3.3, 6.3.4 and 6.3.6.
Worked example
A 6 × 4 m slab at 150 mm, 4,000 psi
- Slab 6 × 4 m at 150 mm, 10% waste.
- 4,000 psi, 3/4 in aggregate, 3 in slump.
- Fineness modulus 2.7, no air entrainment.
- 1
The slab is 6 × 4 × 0.15 = 3.6 m³, and with 10% waste the order is 3.96 m³ — 5.18 cubic yards.
- 2
The water content for a 3 in slump with 3/4 in aggregate is 345 lb per cubic yard.
- 3
The water-cement ratio for 4,000 psi is 0.57, so the cement is 345 / 0.57 = 605 lb.
- 4
Air content is 2%, taking 0.54 ft³ of the 27 in a cubic yard.
- 5
The coarse aggregate is 0.64 × 27 = 17.28 ft³ dry-rodded, or 1,728 lb, taking 10.26 ft³ of solid volume.
- 6
That leaves 7.60 ft³ for the fine aggregate, which at a specific gravity of 2.65 is 1,309 lb.
- 7
The batch is 345 lb water, 605 lb cement, 1,309 lb sand and 1,674 lb stone — 1 : 2.16 : 2.77 : 0.57 by weight.
- 8
For 3.96 m³ that is 1,422 kg of cement, 3,075 kg of sand, 3,933 kg of stone and 811 litres of water.
3.96 m³ · 1,422 kg cement · 811 L water · w/c 0.570.
Interpreting the result
The water-cement ratio is the number that decides the strength; everything else is workability. Adding water to make a mix easier to place raises the ratio and lowers the strength — roughly 1,000 psi for every 0.05 of w/c at this level. If the mix is too stiff on site, the fix is a plasticiser, not a hose.
Assumptions & limitations
- ACI 211.1 absolute-volume method for normal-weight concrete, with the standard water and coarse-aggregate tables.
- Aggregate specific gravities of 2.65 (fine) and 2.70 (coarse), and a dry-rodded coarse bulk density of 100 lb/ft³.
- Batch weights are for oven-dry aggregates; site moisture must be deducted from the added water.
- No admixtures, fly ash or slag are modelled, and no trial batch is implied.
- The waste allowance covers spillage, over-excavation and finishing losses, not over-ordering.
Frequently asked questions
How much waste should I allow?
10% for a slab on grade, 5% for formed work where the dimensions are controlled, and 15% for footings on uneven excavation. Under-ordering means a cold joint; over-ordering means paying for concrete you have to dispose of.
What does air entrainment do?
It creates microscopic bubbles that give freezing water somewhere to expand, which is why exterior concrete in a cold climate needs it. The cost is roughly 1,000 psi of strength at the same cement content, so the mix is usually enriched to compensate.
Is this a final mix design?
No. ACI 211.1 produces a first trial mix. It is confirmed by a trial batch, a slump test and cylinders, and adjusted for the actual aggregate grading, admixtures and the producer's own materials.
Why does more cement make stronger concrete?
Because the water-cement ratio falls. Strength is governed by the ratio, not by the cement content directly, so the way to buy strength is to reduce the water per unit of cement — which usually means more cement at the same workability.
How do I correct for wet sand?
Measure the moisture content of the sand and stone, add their free water to the aggregate weights, and subtract it from the water you add at the mixer. A sand at 5% moisture can carry 60 litres of water per cubic metre, which is enough to turn a good mix into a weak one.
Methodology last reviewed 2026-10-21. Results are informational and not financial or engineering advice — see our disclaimer.