Concrete Patio Calculator
Works out the concrete for a patio at 100 mm, the standard for foot traffic and garden furniture. Also useful as the base for a paved or tiled patio, where the slab is the substrate rather than the finish.
Includes the waste allowance — this is the figure to order.
Share sends a link that reopens these numbers. Copy details pastes the full breakdown as a list.
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 does a concrete patio need to be?
100 mm (4 in) over a compacted sub-base is plenty for foot traffic, furniture and a barbecue. Going thicker adds cost without adding usable strength unless a vehicle will cross it.
How much concrete for a 4 × 3 m patio?
1.2 m³ before waste at 100 mm, or about 1.32 m³ with a 10% allowance — roughly 1.7 cubic yards. Just over the ready-mix minimum, so expect a short-load fee.
Do I need a fall on a patio slab?
Yes — about 1:80, sloping away from the house, so water runs off rather than pooling. It barely changes the volume but it is the difference between a dry patio and a green one.
Can I lay paving on top of this slab?
Yes. If the slab is a base for paving, remember the finished level has to allow for the bedding mortar and the paver thickness — usually another 50–70 mm below your target finish.