Concrete Calculator
Calculate the volume of concrete needed for slabs, footings, columns, or steps. Get results in cubic metres, cubic feet, and bags of ready-mix.
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How to use this calculator
Volume in cubic metres (m³). A standard 25 kg bag of ready-mix concrete covers approximately 0.012 m³; a 40 kg bag covers 0.019 m³.
- 1
Enter the length, width, and depth of your concrete slab or footing in metres.
- 2
A 10% waste allowance is standard — increase for complex shapes.
- 3
Bag counts assume standard pre-mix concrete bags; adjust for your supplier's bag size.
Frequently asked questions
What formula does this concrete calculator use?
Volume = length × width × depth, all in metres, then multiplied by (1 + waste%). A 4 × 3 m slab at 0.15 m deep is 1.800 m³, and with the default 10% waste the tool reports 1.980 m³. Cubic feet are the waste-adjusted volume × 35.3147.
How does it turn volume into bags?
This tool assumes a 25 kg bag yields 0.012 m³ and a 40 kg bag yields 0.019 m³, divides the waste-adjusted volume by each, and rounds up to whole bags. For the 4 × 3 × 0.15 m example that is 165 bags of 25 kg or 105 bags of 40 kg. Those yields are fixed in the tool, so if your product states a different yield, divide the volume by that figure instead.
What is the weight figure based on?
The tool multiplies the waste-adjusted volume by 2,400 kg per m³ and shows tonnes to one decimal place. The 1.980 m³ example comes to 4.8 tonnes. Real density varies with the mix and aggregate, so use it only as a planning figure for delivery or access.
Should depth be entered in metres or millimetres?
Metres. A 150 mm slab is entered as 0.15. Typing 150 would be read as 150 metres deep and produce an enormous volume, so check the unit label before reading the result.
Can it calculate round columns, post holes or sloped slabs?
No. It multiplies three straight dimensions, so it suits slabs, pads and rectangular footings. For a round hole, work out the cylinder volume yourself, or enter a square of the same width as a deliberately generous estimate. A 0.6 × 0.6 × 0.8 m hole comes out at 0.317 m³ with 10% waste. For a slab with a sloping surface, use the average depth.
Does the waste allowance cover everything I might need?
It is a single percentage added to the volume, which is meant for spillage, uneven ground and over-excavation. It does not add reinforcement, formwork, sub-base material or admixtures, and it cannot know about local site conditions.
Is the result enough to order ready-mix?
It gives you a volume to start the conversation with a supplier. Ordering normally rounds up to the supplier’s delivery increments, and the supplier or your designer decides on strength class, slump and any additives for your project.
Does it tell me how thick or strong the concrete must be?
No. This is a quantity estimate only. Slab thickness, mix strength, reinforcement and ground preparation are design decisions governed by local building codes, so ask a qualified professional or your building inspector what applies to your job.
Concrete calculator — volume, bags and weight for a pour
Volume first, bags second
Concrete is bought by volume and, for small jobs, by the bag, so the page produces both from one set of dimensions. The core calculation is a single multiplication of length, width and depth in metres. The waste allowance then scales that volume, and the scaled figure feeds every other line: cubic feet, the number of 25 kg and 40 kg bags, and the approximate weight. Because the bag counts are rounded up from the adjusted volume, you always end on a whole bag rather than a fraction, which is how the product is sold.
Worked examples
The table below shows four rectangular pours run through the calculator. The first is the default 4 × 3 m slab, the second is a longer, thinner 6 × 3 m slab, the third is a small square pit, and the last has the waste allowance set to zero so the bare volume can be seen.
| Length × width × depth | Waste | Volume incl. waste | 25 kg bags | 40 kg bags | Weight |
|---|---|---|---|---|---|
| 4 × 3 × 0.15 m | 10% | 1.980 m³ | 165 | 105 | 4.8 t |
| 6 × 3 × 0.1 m | 5% | 1.890 m³ | 158 | 100 | 4.5 t |
| 0.6 × 0.6 × 0.8 m | 10% | 0.317 m³ | 27 | 17 | 0.8 t |
| 2 × 1 × 0.1 m | 0% | 0.200 m³ | 17 | 11 | 0.5 t |
Why bag counts jump so quickly
The last row shows how rounding behaves. A 0.200 m³ pour divided by 0.012 m³ per 25 kg bag is 16.7 bags, so the tool reports 17. Switching to the 40 kg bag gives 10.5, reported as 11. The first row shows scale: 1.980 m³ needs 165 small bags, which is a lot of mixing and lifting, and it is the main reason the figure is useful as a reality check on whether bags make sense for your pour or whether a delivered load would be easier to handle.
Choosing a waste percentage
The waste field is a plain multiplier, so 10% on 1.800 m³ adds 0.180 m³. Use a higher percentage if the excavation is uneven, the base is soft, or the edges will be formed roughly, and a lower one if the formwork is precise and the base is level. The tool cannot see your site, so the number is your judgement. It is applied to the whole volume, including the weight and bag calculations that follow.
What is not covered
This is a material estimate for rectangular shapes, not a structural design. It does not choose slab thickness, mix strength, reinforcement, joints or ground preparation, and it carries no building-code limits. Those depend on the load, the soil and your local rules, so a qualified professional or inspector decides them. Use the volume to price and order, then confirm the specification with whoever supplies and signs off the work.
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Results are estimates for informational purposes only and do not constitute professional financial, medical, legal, or technical advice. Read full disclaimer →