How to use the Concrete Calculator (Slab Volume)
- Enter the length and width of the slab in metres.
- Enter the thickness in centimetres.
- Enter an allowance for waste and uneven ground.
- Read the volume to order and the approximate weight.
Formula used
Volume = length × width × thickness in metres. Order = volume × (1 + allowance). 1 m³ = 1.30795 cubic yards. Weight ≈ 2.4 tonnes per m³.
Worked example
Here is the Concrete Calculator (Slab Volume) worked through with the example values shown above, so you can check each step by hand.
| Inputs | |
|---|---|
| Length (m) | 5 |
| Width (m) | 3 |
| Thickness (cm) | 10 |
| Allowance for waste | 10 % |
| Result | |
| Concrete to order (m³) | 1.65 |
| Volume with no allowance (m³) | 1.5 |
| Order in cubic yards | 2.1581175 |
| Approximate weight (tonnes) | 3.96 |
The volume calculation
Volume is length × width × thickness, with the thickness in metres. A slab of 5 m by 3 m and 10 cm thick is 5 × 3 × 0.10 = 1.5 m³. With a 10% allowance the amount to order is 1.65 m³, which is 2.16 cubic yards.
Raising the thickness to 15 cm gives 2.48 m³ with the same allowance. A larger 10 m by 6 m slab at 15 cm needs 9.9 m³.
Why add an allowance
Ground is rarely perfectly level, formwork can bulge slightly and some concrete is lost in mixing and spillage. Ordering exactly the calculated volume (1.5 m³ here) leaves no margin, so most people add 5% to 10%, and more for uneven or soft ground.
A little leftover is easier to handle than a shortfall. Ask the supplier about minimum loads and delivery charges for small amounts.
Thickness and strength
The right thickness depends on the use. Garden paths and patios are often around 10 cm, while driveways, garage floors and anything carrying vehicles need more, and often steel reinforcement too. The grade of concrete matters as well.
This calculator gives volume only. For structural slabs, follow an engineer’s design or local building rules.
Weight and mixing
Concrete weighs about 2.4 tonnes per cubic metre, so the example is about 4 tonnes. That matters for access and for the base it sits on.
If you mix your own, the cement, sand and aggregate proportions depend on the grade, which this calculator does not work out. Ready-mix delivery avoids the effort and gives a consistent quality for larger pours.
Before you order
Check the formwork is level and braced, and that the base is compacted, because the slab is only as good as what is under it. Measure the depth at several points, not just one, and use the average thickness in the calculator.
Plan for the pour: have enough people and tools ready, because concrete starts to set within hours. Hot or cold weather changes the setting time and the curing care it needs, so ask your supplier for advice on the day.
Frequently asked questions
How much extra should I order?
Usually 5% to 10%, more on uneven ground.
How do I convert cubic metres to cubic yards?
Multiply by 1.30795. The calculator shows both.
Can I use it for a circular slab?
Not directly. Use the area from the circle calculator, then multiply by thickness.
How thick should a driveway be?
Thicker than a path, and usually reinforced. Ask a builder or follow local guidance.
Does it give bags of cement?
No. Mix proportions vary with grade, so ask your supplier.
How much does concrete weigh?
About 2.4 tonnes per cubic metre when set.
This calculator gives estimates for information only. Actual figures from a bank, employer, tax authority or doctor may differ. It is not financial, tax, legal or medical advice.