How Much Concrete Do I Need for a Slab, Footing or Column – m³ and Cement Bags
Short answer: a 6 × 4 m slab that is 12 cm thick needs 2.88 m³ of concrete, and with a 5 % allowance you should order about 3.1 m³. If you mix C25/30 concrete by hand, that is about 43 bags of cement (25 kg), 3.9 m³ of aggregate and 544 litres of water. For 10 fence post footings of 30 × 30 × 80 cm, 0.8 m³ is enough, which is 10 bags of cement for C20/25. The concrete calculator works this out for a slab, strip foundation, foundation footing, column, beam and stairs.
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When you need this calculation
You need this calculation before ordering from a ready-mix plant or buying cement, to know how many cubic metres your slab, footing or column really needs. On a building site the two most expensive mistakes are too little concrete, when the truck leaves, the slab is left unfinished and you end up with a cold joint, and too much concrete, when you pay for cubic metres that end up in a skip or on the ground.
The calculator adds up the volume of all elements, adds the allowance you choose, rounds the quantity to order and estimates the weight of the concrete. If you mix concrete yourself, it also calculates cement in bags, aggregate and water for the selected concrete class. You can save the result to a project, send it to your contractor or export it as a PDF materials list.
Who the calculator is for
- Homeowners and DIYers building a garage, patio, fence or shed base themselves who want to know how much concrete to order.
- Contractors who need a quick quantity for a quote or a ready-mix order, without working it out by hand on paper.
- Anyone mixing concrete by hand or with a mixer who needs the number of cement bags and the amount of aggregate.
- Cost planning: with a known price per cubic metre or per bag of cement you immediately see a rough cost of the concrete.
How to use the concrete calculator – step by step
- 1. Add an element: slab, strip foundation, foundation footing, column, beam or stairs. You can add up to 20 elements to one calculation.
- 2. Enter the dimensions in metres or centimetres and the number of identical elements (e.g. 6 identical foundation footings).
- 3. If an element has an opening, enter it – its volume is subtracted.
- 4. Choose the allowance: 5 % for flat formwork and pumping, 10 % for uneven excavation or manual placing, or your own percentage.
- 5. Choose how to round the quantity (0.01, 0.1 or 1 m³) to match how you order.
- 6. If you mix it yourself, turn on hand mixing and choose the concrete class – you will get cement, aggregate and water.
- 7. Save the calculation to a project or export a PDF for the ready-mix plant and the contractor.
Formula: how concrete volume is calculated
The basic formula is simple: volume = length × width × height (thickness). For a round column, volume = π × r² × height, where r is the radius. When you have several elements, the volumes are added together and the volume of openings is subtracted. Then the allowance is added: volume to order = volume × (1 + allowance). The weight of concrete is approximately volume × 2400 kg/m³.
It is important to convert all measurements to metres before multiplying. A slab 12 cm thick has a height of 0.12 m, not 12 – this is the most common cause of a result that is a hundred times too large.
Calculation examples
Garage slab 6 × 4 m, 12 cm thick: 6 × 4 × 0.12 = 2.88 m³. With a 5 % allowance: 2.88 × 1.05 = 3.02 m³; rounded to 0.1 m³ you order 3.1 m³. The concrete weighs about 7.3 tonnes.
Strip foundation 10 m long, 40 cm wide, 50 cm high: 10 × 0.4 × 0.5 = 2 m³; with a 5 % allowance that is 2.1 m³.
Round column 40 cm in diameter and 3 m high: π × 0.2² × 3 = 0.377 m³; with a 5 % allowance about 0.4 m³. For small quantities like this it is often more cost-effective to mix concrete on site.
How much cement, aggregate and water for hand mixing
For hand mixing the calculator uses approximate mix designs for each concrete class, per cubic metre of finished concrete. Sand and gravel are shown together as aggregate, because that is how they are usually bought for mixing. Cement is rounded up to whole 25 kg bags.
| Concrete class | Use | Cement per m³ | 25 kg bags per m³ | Aggregate per m³ | Water per m³ |
|---|---|---|---|---|---|
| C16/20 | light foundations, non-load-bearing elements | 280 kg | 11.2 | 1.3 m³ | 160 L |
| C20/25 | foundations, walls, floor slabs | 320 kg | 12.8 | 1.3 m³ | 170 L |
| C25/30 | slabs, columns, beams, load-bearing elements | 350 kg | 14 | 1.3 m³ | 180 L |
| C30/37 | heavy-load elements | 400 kg | 16 | 1.3 m³ | 185 L |
Ready-mix or hand mixing?
For slabs, foundations and anything larger than roughly 1 m³, concrete is normally ordered from a ready-mix plant: the quality is consistent, the strength class is controlled and placing is fast. With a pump, expect an extra charge and keep in mind that the pump uses some concrete to fill the line, so an allowance of at least 5 % is useful when pumping. Also ask the plant about the minimum quantity and order increment, because many round up to half a cubic metre.
Hand mixing makes sense for small quantities: fence posts, a garden shed base, repairs and small jobs. Load-bearing elements of a house are governed by the structural design and quality requirements, so ready-mix concrete is used for them.
The most common mistakes when calculating concrete
- No allowance – the last part of the slab runs short of concrete and the second truck arrives too late.
- Thickness entered in centimetres without converting to metres.
- Forgotten elements: footing widenings, stairs or lintels.
- Uneven excavation calculated as flat formwork – for uneven ground use a 10 % allowance.
- Confusing volume and weight: 1 m³ of concrete weighs about 2400 kg, so a van or trailer cannot carry a cubic metre of concrete.
- Treating approximate hand-mix designs as valid for structural concrete – the structural design and the supplier decide.
What else to calculate along with concrete
Concrete almost always needs rebar too: the rebar calculator works out the number of bars, stirrups and kilograms of steel for a foundation or slab. For digging foundations and hauling away soil use the excavation and fill calculator, and for irregular shapes the volume calculator. All calculations can be saved to the same project and exported as a single PDF materials list.
Frequently asked questions
How much concrete do I need for a 6 × 4 m slab?
At 12 cm thick the volume is 2.88 m³; with a 5 % allowance order about 3.1 m³. At 15 cm thick the volume is 3.6 m³, and with the allowance about 3.8 m³.
How many bags of cement do I need for a 6 × 4 m slab?
With a thickness of 12 cm and a 5 % allowance (3.02 m³), C25/30 needs 1058 kg of cement, which is 43 bags of 25 kg, 3.9 m³ of aggregate and 544 L of water. For C20/25 it is 39 bags.
How much concrete and cement do I need for fence posts?
For 10 foundation footings of 30 × 30 × 80 cm: 0.72 m³, and with a 5 % allowance you order 0.8 m³. Mixing C20/25 by hand, that is 10 bags of cement, 0.98 m³ of aggregate and about 129 L of water.
What should I round a ready-mix order up to?
The calculator offers 0.01, 0.1 or 1 m³ (default 0.1). For 3.02 m³ that is 3.1 m³, and with a 1 m³ step 4 m³. Ask the plant about the minimum quantity and the order step.
Is the concrete calculation free?
Yes. The calculator is free on the web and in the Android app, where calculations are saved to projects and exported to PDF.
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