Why multiply the wet volume by a dry factor?

Dry cement, sand, and gravel have air voids that fill when water is added, so the loose dry material occupies more space than the finished concrete. A dry-volume factor of about 1.54 converts wet to dry volume before splitting it by the ratio.

How is each material's quantity found?

The dry volume is divided into parts by the ratio. For 1:2:4 there are 7 parts: cement is 1/7, sand 2/7, gravel 4/7 of the dry volume. Each is then multiplied by its bulk density to get a weight.

How is the water quantity calculated?

Water is the water-cement ratio times the cement weight. A 0.5 water-cement ratio with 320 kg of cement needs 160 litres of water. Lower ratios give higher strength but reduce workability.

What strength do common ratios give?

Approximate 28-day cube strengths are: 1:3:6 about 10 MPa, 1:2:4 about 20 MPa, 1:1.5:3 about 25 to 30 MPa, and 1:1:2 about 35 MPa. Actual strength depends on the water-cement ratio, materials, and curing.

Are these nominal or design mixes?

These are nominal volume-batched mixes suitable for small works. For structural concrete a designed mix tested to the specified characteristic strength, batched by weight, is required by codes such as ACI 318 and Eurocode 2.

Concrete Mix Ratio Calculator

Volume-batched concrete is specified as a ratio of cement to sand to gravel, such as 1:2:4. To order materials you need to turn that ratio plus a target volume into actual quantities of each ingredient. This calculator does that, accounting for the air voids in dry material and your chosen water-cement ratio.

How it works

The finished wet volume is scaled up to a dry volume, then split by the ratio:

dry volume = wet volume × 1.54
parts      = cement + sand + gravel
cement vol = dry volume × cement / parts   (repeat for sand, gravel)
weight     = volume × bulk density
water      = water-cement ratio × cement weight

Typical bulk densities used are about 1440 kg/m³ for cement, 1600 kg/m³ for sand, and 1450 kg/m³ for gravel. A 50 kg cement bag is roughly 0.0347 m³.

Example and notes

For 1 m³ of 1:2:4 concrete the dry volume is 1.54 m³ split into 7 parts: cement 0.22 m³ (about 317 kg, just over six 50 kg bags), sand 0.44 m³, gravel 0.88 m³. At a 0.5 water-cement ratio that is about 158 litres of water. Lower the water-cement ratio for strength but expect stiffer, less workable concrete; raise it only with a plasticiser, never by simply adding water on site.