Concrete calculator
Estimate the concrete required for a slab, footing, wall or cylindrical posts, including waste, mix dosage and packaging.
Concrete calculation
Describe your project: volume, waste allowance and purchase quantities are recalculated automatically.
Supply settings
Indicative quantities
Calculation details
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Mix proportions and yields are quantity estimates. For the mix design, strength, thickness or reinforcement of a structure, follow the manufacturer’s instructions and the applicable technical rules.
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How does this tool work?
The concrete calculator works out the volume needed for a slab, a strip footing, a wall or a set of cylindrical posts. It adds a site waste allowance and, depending on how the concrete is supplied, turns that volume into an order for a ready-mix plant, indicative cement, sand, gravel and water quantities, or a number of premixed bags.
Values entered in centimetres (thickness, depth, diameter) are converted to metres first. The engine then applies the formula of the selected shape: length × width × thickness for a slab, length × width × depth for a footing, length × height × thickness for a wall and count × π × (diameter ÷ 2)² × depth for posts.
The waste allowance is applied to the raw volume: final V = raw V × (1 + allowance ÷ 100). It covers an uneven sub-base, spillage during the pour and the concrete left in the chute or mixer.
For ready-mix concrete, the figure to use is that final volume, to be confirmed with the plant. When you mix on site, the selected dosage (200 to 450 kg of cement per m³, interpolated between the 250, 300, 350 and 400 kg/m³ presets) gives indicative cement, sand, gravel and water quantities, and the number of cement bags is rounded up.
For premixed bags, the final volume in litres is divided by the yield of one bag and rounded up to a whole bag.
Example: a 5 m × 4 m slab that is 12 cm thick represents 2.40 m³ before waste. The calculator converts that geometry into a quantity; mix design, strength and reinforcement still need to be checked for the actual project.
How to use this tool
Choose the shape
Slab, footing, wall or cylindrical posts.
Enter the dimensions
Measure the actual area to fill, including any extra depth where the ground is uneven.
Choose the supply
Ready-mix delivery, mixing on site or premixed bags.
Check the assumptions
Adjust the allowance, dosage, cement bag weight or bag yield to the product you will actually use.
Concrete volume formulas
The calculation starts from the geometry of the pour, then applies the waste allowance.
Slab
V = length × width × thickness
All dimensions are converted to metres to obtain m³.
Cylindrical posts
V = count × π × (diameter / 2)² × depth
Each post hole is treated as a cylinder.
Volume to order
final V = raw V × (1 + allowance / 100)
The allowance covers small losses and site tolerances.
This calculation estimates quantities. It does not size the strength, thickness or reinforcement of a structure.
Use cases
Patio or garage slab
Turn length, width and thickness into cubic metres to order, with an adjustable allowance.
Strip footing
Use the trench dimensions to estimate the volume of a foundation footing.
Fence or deck posts
Calculate several cylindrical post holes at once from their diameter and depth.
Comparing supply options
Compare a ready-mix delivery, mixing on site and premixed bags for the same pour.
Examples
Ready-mix patio slab
5 × 4 × 0.12 = 2.4 m³, then 2.4 × 1.10 = 2.64 m³: this is the volume to confirm with the plant.
Input
Slab 5 m × 4 m × 12 cm · 10% allowance · ready-mix concreteResult
Area 20 m²; raw volume 2.4 m³; concrete required 2.64 m³Same slab mixed on site
Each quantity of the per-m³ mix is multiplied by 2.64 m³: 350 × 2.64 = 924 kg of cement, i.e. 26.4 bags rounded up to 27.
Input
Slab 5 m × 4 m × 12 cm · 10% allowance · 350 kg/m³ dosage · 35 kg cement bagsResult
Cement 924 kg (27 bags); sand 2,112 kg; gravel 2,772 kg; water 462 L10 posts with premixed bags
One post holds π × 0.15² × 0.5 ≈ 0.0353 m³. With the allowance, 388.8 L ÷ 15 L = 25.9 bags, rounded up to 26.
Input
10 posts Ø 30 cm × 50 cm · 10% allowance · 15 L yield per bagResult
Raw volume 0.35 m³; concrete required 0.39 m³; 26 bagsTips and best practices
- Measure the thickness at several points when the sub-base is not perfectly level: a slab that averages 13 cm instead of 12 cm needs about 8% more concrete.
- For premixed bags, use the yield printed on the bag rather than a generic value; it varies with the product and the bag size.
- A reasonable allowance prevents running short at the end of a pour, but an excessive one adds cost and waste.
Sources and references
Frequently asked questions
How much concrete do I need for a slab?
Multiply length, width and thickness in metres. A 5 m × 4 m slab that is 12 cm thick holds 2.4 m³; add an allowance suited to the site, for example 10%, giving 2.64 m³.
How many cement bags are needed for 1 m³ of concrete?
It depends on the dosage and the bag weight. At 350 kg of cement per m³, that is 10 bags of 35 kg per cubic metre in theory, before multiplying by the total volume and rounding up.
What waste allowance should I use?
The calculator suggests 10% by default. The right value depends on how accurate the measurements are, how even the ground is, expected losses and the delivery method.
Does the calculator size a foundation?
No. It only estimates a quantity of concrete from the dimensions you enter. Structural design must follow the technical rules and requirements that apply to the project.
Does the suggested dosage replace a concrete mix design?
No. The quantities shown are indicative. A real mix depends on the aggregates and their moisture, the target strength and the site specification.
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