Concrete is one of the few construction materials where getting the quantity wrong has immediate, potentially irreversible consequences. Order too little and you're scrambling while the slab starts to set. Ready-mix lorries don't wait — they tip, they go, and you deal with the consequences. Here's how to calculate it properly before anyone picks up a phone.
The Core Formula
Volume (m³) = Length × Width × Depth (all in metres). Our concrete calculator handles this instantly for slabs, paths and post holes. Combine with our area calculator for base dimensions.
Example: Patio Slab
4m × 3.5m × 100mm deep: convert depth to metres (0.1m). Volume = 4 × 3.5 × 0.1 = 1.4 m³. Add 10% wastage: 1.54 m³. Order 1.6 m³.
Recommended Depths
- Pedestrian path: 75-100mm
- Patio: 100mm
- Driveway (cars): 150mm minimum
- Garage floor: 150mm
- Fence post footing: 600mm deep, diameter ≈ 3× post width
- House extension foundations: 300mm+ — always consult a structural engineer
Ready-Mix vs Mixing Yourself
Ready-mix: essential above 0.3 m³. Minimum order typically 0.5-1 m³. Costs £80-£150/m³ plus delivery. Ensure good lorry access before booking. Self-mix: practical for small pours. Standard ratio: 1 cement : 2 sand : 3 aggregate. One 25kg cement bag ≈ 0.01 m³ finished concrete.
Sub-Base: Do Not Skip
A compacted hardcore sub-base (100-150mm crushed stone) is essential under any driveway or patio in UK conditions — particularly on clay soil. Without it, ground movement will crack the slab within a few winters. Calculate sub-base volume separately using the same formula.
Have Everything Ready Before the Lorry Arrives
Ready-mix begins setting within 2 hours of batching. Formwork installed, sub-base compacted, reinforcement placed, helpers present — all before ordering. Concrete will not wait while you rummage for a screed board.
Further reading: British Precast provides technical guidance on domestic concrete specification. Visit British Precast for concrete guidance.