Concrete Mix Design Calculator
Work out cement, sand, gravel and water per cubic metre from the target grade and water-cement ratio.
Inputs
Result
Formula
| Target mean strength | fcu,0 = fcu,k + 1.645σ (σ = 5.0 MPa) |
| Effective cement strength | fce = γc × cement class |
| Water-cement ratio | W/C = 0.53fce ÷ (fcu,0 + 0.53×0.20×fce) |
| Cement content | mc = mw ÷ (W/C) |
| Total aggregate | ms + mg = 2400 − mw − mc |
| Sand | ms = (ms+mg) × sand ratio |
C30 with 42.5 class cement, 31.5 mm aggregate, 90 mm slump and 38% sand: W/C 0.527, water 199 kg, cement 377 kg, sand 693 kg, gravel 1131 kg — a ratio of 1 : 1.84 : 3.00.
Reference mixes by grade (per m³, medium sand, 31.5 mm aggregate)
| Grade | W/C | Cement (kg) | Sand (kg) | Gravel (kg) | Water (kg) |
|---|---|---|---|---|---|
| C20 | 0.62 | 290 | 750 | 1150 | 180 |
| C25 | 0.57 | 320 | 715 | 1155 | 182 |
| C30 | 0.53 | 377 | 693 | 1131 | 199 |
| C35 | 0.48 | 400 | 680 | 1125 | 192 |
| C40 | 0.44 | 440 | 665 | 1118 | 194 |
FAQ
How do water-cement ratio and slump relate?
Water-cement ratio governs strength; slump governs workability, and the two pull against each other. For a given grade, a wetter mix either means more water (lower strength) or a superplasticiser (same strength, higher cost). Adding water on site is the worst trade: you save a few yuan of admixture and lose a whole strength grade.
Why is my cement figure higher than the ready-mix supplier's?
Suppliers substitute fly ash, slag and admixtures, cutting cement by 15-30% while keeping total binder constant. This calculator assumes pure cement. With 20% fly ash, multiply cement by 0.8 and add fly ash at about 0.25 times the cement mass to hold the binder content.
What sand ratio should I use?
Sand ratio trades workability against strength: too high loses strength, too low segregates and blocks the pump. Typical ranges for crushed stone: 33-38% at 30-70 mm slump, 38-43% at 70-120 mm, 40-45% for pumped concrete at 120-180 mm. Use the low end for fine sand and the high end for coarse sand; manufactured sand runs 2-3 points higher than natural sand.
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Results are estimating aids only and are not a design basis. Construction and final accounts must follow the drawings, the applicable standards and an approved method statement. Anything affecting structural safety, shoring or falsework must be checked by a suitably qualified engineer.