Concrete and mortar mix design
Cement, sand, gravel and water for a volume of concrete or mortar, by the dry-volume method.
How it is calculated
Loose aggregate (before mixing) takes up more volume than the compacted mix, because there is air between the particles. To get 1 m³ of concrete placed in the work, you need to dry-mix about 1.54 m³ of loose cement + sand + gravel; for 1 m³ of mortar, about 1.33 m³ of loose cement + sand. This is the "dry-volume method", used on site with buckets or wheelbarrows to dose material without scales.
Within that dry volume, each material takes the share set by the mix ratio. For example, in a 1:2:3 concrete (one part cement, two of sand, three of gravel) the parts add up to 6: cement takes 1/6 of the dry volume, sand 2/6 and gravel 3/6.
The cement's weight comes from its bulk density, about 1440 kg/m³ (loose cement from a bag weighs less per m³ than compacted powder). The number of bags is always rounded up: you end up with a little extra cement, never short halfway through the job.
Water = cement (kg) × water/cement ratio; 1 kg of water equals 1 litre.
Example with the default values: 1 m³ of 1:2:3 concrete, 25 kg bag, water/cement ratio 0.5. Dry volume = 1 × 1.54 = 1.54 m³. Sum of parts = 1+2+3 = 6. Cement = 1.54 × 1/6 = 0.2567 m³ → 0.2567 × 1440 = 369.6 kg → 369.6 / 25 = 14.78 → 15 bags. Sand = 1.54 × 2/6 = 0.513 m³. Gravel = 1.54 × 3/6 = 0.77 m³. Water = 369.6 × 0.5 = 184.8 L.
Keep in mind
- These mix figures are reference values for minor works without lab testing; for elements with a structural role (columns, beams, slabs), the exact mix design and concrete strength must be set by a structural calculation, not by this calculator.
- Water is the factor that most affects strength: more water means weaker concrete or mortar, more prone to cracking as it dries. Add water gradually: it is easier to correct a shortfall than an excess.
- Cement bags spoil with damp within a few weeks; store them raised off the ground and well sealed, and size the stock to how long you will actually take to use it.
- Damp sand and gravel bring their own water: if they are wet, cut the added water by eye (roughly 20-40 L less per m³ of concrete with very damp aggregate) and check it by mixing a small test batch.