Concrete Slab Calculator
Enter the slab dimensions to get the volume of concrete to order, in both cubic metres and cubic yards, along with the bag count if you are mixing on site and the finished weight.
Includes the waste allowance — this is the figure to order.
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The formula
volume = length × width × thickness × (1 + waste)
A slab is a rectangular prism, so the volume is the three dimensions multiplied together. The only judgement in it is the waste allowance — concrete cannot be topped up once it starts curing, so under-ordering is far more expensive than over-ordering.
What to watch out for
The arithmetic is straightforward. What catches people out is everything around it:
- Order by volume, not by area. Thickness is where the cost is. Going from 100 mm to 150 mm on a 20 m² slab adds a full cubic metre.
- Always add waste. A sub-base is never perfectly level, and the low spots take more concrete than the drawing says. 10% is standard; 15% on rough or uncompacted ground.
- Ready-mix has a minimum. Most suppliers will not deliver under 1 m³ (about 1 yd³), and charge a short-load fee below roughly 3 m³. Below the minimum you pay for the minimum.
- Bags stop making sense quickly. One cubic metre is about 80 × 25 kg bags — two tonnes to carry, open and mix by hand, and it has to be placed in one continuous pour to avoid cold joints.
Frequently Asked Questions
How thick should a concrete slab be?
100 mm (4 in) is standard for a patio, path or shed base. 150 mm (6 in) for a driveway or anything carrying vehicles. Heavier loads need a structural design, not a rule of thumb.
How many bags of concrete are in a cubic yard?
About 45 × 80 lb bags, 60 × 60 lb bags, or 90 × 40 lb bags. In metric, a cubic metre is roughly 80 × 25 kg bags.
Should I order ready-mix or use bags?
Under about 0.5 m³ (0.65 yd³), bags are usually cheaper and simpler. Above 1 m³, ready-mix almost always wins on both cost and effort — and it arrives as one consistent batch, which matters for strength.
How much does a concrete slab weigh?
Around 2,400 kg per cubic metre, or about 4,050 lb per cubic yard. A 6 × 3 m slab at 100 mm weighs roughly 4.3 tonnes.
What waste allowance should I use?
10% for a prepared, level sub-base. 15% if the ground is uneven, the formwork is rough, or you are pouring against an existing surface.