Concrete Mix Ratio Material Calculator

What does this tool do?

Concrete mixed on site by hand or in a small mixer is usually batched by volume: one measure of cement, two of sand, four of gravel. For a ratio you provide, this tool estimates how many bags of cement and how much sand, gravel and water a given concrete volume needs.

Important: this is not a mix design. The strength a ratio delivers depends on the cement, the aggregate, the water, compaction and curing, and only laboratory testing can establish it. Use certified ready mixed concrete for structural members.

How is it calculated?

Dry materials measured loose shrink when they are mixed and compacted, because fine particles fill the gaps between coarse ones. The required concrete volume is therefore multiplied by a dry volume factor (site practice uses about 1.54). The dry volume is then shared out by the ratio:

cement volume = V × k × c / (c + s + a)

The cement volume becomes kilograms through the bulk density and bags through the bag weight. Water is the cement mass times the water to cement ratio.

Example

For 1 m³ of concrete, a 1 : 2 : 4 ratio and a factor of 1.54, the dry volume is 1.54 m³. Cement is 1.54 / 7 = 0.22 m³, which is 0.22 × 1440 = 316.8 kg or 7 bags of 50 kg. Sand is 0.44 m³, gravel 0.88 m³ and, at a water to cement ratio of 0.5, water is 158 litres.

Limits

The factor and the densities depend on the materials, so the result is approximate. Deduct the moisture in wet sand from the water. Damp sand also bulks, which adds error to volume batching. Large and structural pours use a designed mix batched by mass.