How Much Concrete for a Slab?
Quick Answer
1.2m³ of concrete for a 4m × 3m slab at 100mm depth
Based on: length × width × depth. Add 10% for waste and over-dig.
How We Calculated This
- Length: 4m
- Width: 3m
- Depth: 100mm = 0.1m
- Volume: 4 × 3 × 0.1 = 1.2m³
- With 10% waste: 1.32m³
Common Slab Depths
- 75mm: Light-duty paths, greenhouse bases
- 100mm: Standard domestic — sheds, patios, garage floors
- 150mm: Driveways, heavy loads, workshop floors
- 200mm+: Commercial/industrial, structural engineer design
Ready-Mix vs Site-Mixed
- Ready-mix delivery: Ideal for 1m³+. Minimum order typically 1m³. Cost: £80–120/m³ (2026)
- Volumetric mixer: Pay for what you use, ideal for 2–5m³
- Site-mixed: Suitable for under 1m³. Use bags of cement + ballast or pre-mixed bags
- Pre-mixed bags (20kg): You need approximately 110 bags per m³ — expensive for large slabs
Preparation
Always pour onto a compacted sub-base (100mm Type 1 MOT minimum). Use DPM (damp-proof membrane) under internal slabs. Edge formwork should be level and securely braced. Reinforce with A142 or A193 mesh for driveways and structural slabs.
Last updated: April 2026. Volumes are net — always add 10% for waste, over-dig, and uneven ground.