Getting the concrete quantities right for a slab is critical — too little and you're making emergency mixes or having awkward cold joints; too much and you're paying for concrete you can't use. This guide walks through exactly how to calculate materials for any concrete slab, from a small garage floor to a large patio.
Choosing the Right Thickness
Slab thickness is the most important variable — it affects structural performance and material quantities dramatically. Getting this wrong is a common and expensive mistake.
| Application | Min. Thickness | Recommended | Notes |
|---|---|---|---|
| Garden path (pedestrian) | 75 mm | 100 mm | No reinforcement needed |
| Patio / terrace | 100 mm | 125 mm | Add reinforcement mesh |
| Shed / outbuilding base | 100 mm | 150 mm | A142 mesh recommended |
| Driveway (cars) | 150 mm | 175 mm | Reinforcement essential |
| Garage floor | 150 mm | 175 mm | DPM + reinforcement |
| Van / light goods vehicle | 175 mm | 200 mm | Engineer design advisable |
volume-formula">The Concrete Volume Formula
Volume (m³) = Length (m) × Width (m) × Thickness (m)
Add 10% waste factor: Order volume = Volume × 1.10
Thickness in metres: 150 mm = 0.150 m, 100 mm = 0.100 m
Worked Example: 4m × 3m Garage Floor, 150mm Thick
- Volume: 4 × 3 × 0.15 = 1.8 m³
- Add 10% waste: 1.8 × 1.10 = 1.98 m³ → order 2.0 m³
- Mix materials for 1:2:4 (C20 grade):
- Cement: 2.0 × 320 kg/m³ = 640 kg ≈ 12.8 bags (50 kg bags)
- Sand: 2.0 × 720 kg/m³ = 1,440 kg ≈ 1.44 tonnes
- Aggregate: 2.0 × 1,200 kg/m³ = 2,400 kg ≈ 2.4 tonnes
Mix Ratios for Different Applications
| Grade | Mix Ratio | Strength | Application |
|---|---|---|---|
| C15 | 1:3:6 | 15 N/mm² | Blinding, non-structural |
| C20 | 1:2:4 | 20 N/mm² | Paths, patios, shed bases |
| C25 | 1:1.5:3 | 25 N/mm² | Garage floors, driveways |
| C30 | 1:1:2 | 30 N/mm² | Heavy-duty, vehicle loads |
Ready-Mix vs Site-Mixed: Which Is Better?
For volumes over 1 m³, ready-mix concrete from a local supplier is almost always better value and quality. You specify the grade and slump class; the supplier delivers mixed concrete. For smaller jobs, bagged cement products (like Postcrete or Rapid Set) are convenient but costly at scale. A rough guide:
- Under 0.5 m³: bagged products acceptable
- 0.5–1.5 m³: consider hiring a mixer and buying materials separately
- Over 1.5 m³: ready-mix is almost certainly cheaper and better
UK ready-mix prices (2025): approximately £120–£160/m³ for C25 concrete, delivered. Minimum order is typically 0.5 m³; short-load charges apply below 3 m³.
Essential Preparation Steps Before Pouring
- Excavate to correct depth: Slab depth + 100 mm hardcore base + any DPM
- Compact hardcore: Uncompacted hardcore will cause slab to settle unevenly
- Install DPM: 1200-gauge polythene damp-proof membrane for any covered structure
- Set formwork level: Timber shuttering at your target slab height — check with a spirit level
- Place reinforcement mesh: A142 mesh on 25 mm cover spacers for driveways and garage floors