What this calculator works out
This calculator multiplies length, width and depth to give the volume of concrete a slab needs, then adds a waste allowance and shows the result in cubic metres and litres.
Concrete is ordered and mixed by volume, and running short mid-pour is the one outcome worth designing against. A slab poured in two sessions has a cold joint through it and never has the strength of a single pour.
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Before you order
- Measure the excavation, not the finished slab. Ground is rarely level and the deep corner sets the volume.
- Depth is in millimetres here. A typical shed base or path is 100 mm; a drive taking vehicles needs 150 mm or more over a proper sub-base.
- Allow at least 10% waste. Ground irregularity, spillage and over-dig routinely account for that much.
- Ready-mix is sold in fixed increments, commonly 0.5 m³. Order to the increment above your figure, not below.
100 mm suits foot traffic, sheds and paths. 150 mm is the usual minimum for a car, over a compacted sub-base. Reinforcement, ground conditions and anything structural change the answer entirely and are a job for a structural engineer rather than a rule of thumb.
How this calculator works
A volume, then an allowance:
Base volume = length × width × (depth ÷ 1,000)
Required = base volume × (1 + waste ÷ 100)
Litres = cubic metres × 1,000Dividing depth by 1,000 converts millimetres to metres so all three dimensions share a unit. One cubic metre is 1,000 litres, which is a useful cross-check when comparing against bagged products quoted in litres.
The calculation gives finished, placed concrete. If you are mixing from raw materials, the dry ingredients occupy about 54% more volume — the Concrete Mix Calculator handles that step.
Worked example: a 4 m by 3 m slab
Using the default figures — 4 m by 3 m at 100 mm deep with a 10% allowance:
- Base volume: 4 × 3 × 0.1 = 1.2 m³
- With waste: 1.32 m³, or about 1,320 litres
Ready-mix would be ordered as 1.5 m³, the next half-metre increment. Mixing that by hand is roughly 60 bags of cement plus aggregate and several hours of work — which is why 1.32 m³ is around the point where most people stop mixing and start phoning. A mixer lorry also delivers a single consistent batch, which a day of barrow loads does not.
Common mistakes
- Measuring the finished slab rather than the excavation. The deepest point sets the volume.
- Entering depth in metres. The field expects millimetres.
- Ordering exactly the calculated figure. Ready-mix comes in increments; round up.
- Pouring 100 mm for a driveway. Vehicles need more, over a proper sub-base.
- Ignoring the sub-base. Concrete laid straight onto soft ground cracks regardless of its thickness.
Frequently asked questions
How many bags of ready-mixed concrete is a cubic metre?
Roughly 50 to 60 of the 20 kg bags sold at builders' merchants, since each yields around 0.01 m³. That is a substantial amount of mixing and a lot of bags to move. Bagged mix suits small jobs such as fence posts and repairs; for anything approaching a cubic metre, ordering ready-mix or hiring a mixer is usually both cheaper and better.
Do I need reinforcement?
For a path or a shed base on stable ground, generally not. Steel mesh is normal for driveways and larger slabs, and it needs positioning in the upper third of the slab on spacers rather than laid on the ground where it does nothing. Anything structural, or any slab supporting a building, should be designed rather than estimated.
What about expansion joints?
Concrete moves with temperature and shrinks as it cures. Large slabs need movement joints, commonly every 4 to 5 metres and where a slab meets a wall or another slab. Without them, the concrete will crack where it chooses rather than where you planned.
Can I pour concrete in cold weather?
Below about 5°C the curing reaction slows sharply, and freezing before it has gained strength permanently damages the concrete. Below 3°C most suppliers advise against pouring without protection. Hot weather has the opposite problem — rapid drying causes surface cracking, so the slab needs covering and keeping damp.
Is what I enter stored?
No. Measurements are processed in your browser and never transmitted or retained.
Related tools
References
- GOV.UK — building regulations guidance on foundations and ground floors
- Health and Safety Executive — cement safety, including skin burns and silica dust
Sources are checked at publication and can change — how I choose and check references.