Concrete and materials calculator
Concrete volume plus the cement, sand, gravel and water needed.
Calculator inputs
Results
Enter your values and press “Calculate” to see the result.
In short
- What it calculates
- Concrete volume plus the cement, sand, gravel and water needed.
- Formula used
Volume = length × width × thickness × (1 + waste)- Example
- A 5 × 3 m slab, 15 cm thick, standard mix and 5% waste:
Concrete calculator
For a floor slab, a footing or a base, the first thing you need is the volume; then how many bags of cement and how much aggregate to buy.
The waste allowance covers spillage, uneven ground and what stays in the mixer.
How it works
Volume is length by width by thickness, increased by the waste allowance. Cement quantities come from the chosen mix strength in kilos per cubic metre.
Formula
Volume = length × width × thickness × (1 + waste)
Cement = volume × mix strength
Sand ≈ volume × 0.5 m³
Gravel ≈ volume × 0.75 m³
Water ≈ cement × 0.55 L
Worked example
A 5 × 3 m slab, 15 cm thick, standard mix and 5% waste:
Volume: 2.363 m³
Cement: 708.75 kg → 15 bags of 50 kg
Sand: 1.18 m³ · Gravel: 1.77 m³
Explanation
The water-cement ratio governs everything else
It is the factor that most determines final strength. Adding extra water makes concrete easier to work and much weaker, because the surplus water leaves pores as it evaporates. A small excess can halve the strength, and it is the commonest error on small jobs.
Traditional mixes
A 1:2:3 proportion means one part cement, two of sand and three of gravel, measured by volume. It is a reasonable reference for general purpose concrete, but structural elements must follow the project specification rather than a rule learned by ear.
Curing matters as much as mixing
Concrete does not dry: it sets through a chemical reaction that needs water. Letting it lose moisture in the first days, especially in sun or wind, prevents it reaching its strength. Keeping it damp for a week can make a noticeable difference to the result.
Always order extra
When calculating volume, add between five and ten per cent for ground irregularities, spillage and formwork that gives. Running short mid-pour creates a cold joint, which is a permanent weak point.
Frequently asked questions
Which mix strength do I need?
For slabs and paths the standard 300 kg/m³ is enough. Structural elements need 350 kg/m³ or more, as specified by the design.
Does the water-cement ratio matter?
A great deal. Extra water makes the mix easier to work but noticeably reduces final strength.
Why should I not add water to work it more easily?
Because surplus water leaves pores as it evaporates and greatly reduces strength. Workability is improved with admixtures, not more water.
How much extra concrete should I order?
Between five and ten per cent. Running short mid-pour creates a cold joint that permanently weakens the element.
Need to calculate something else?
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