How to use the concrete slab calculator
- Pick rectangle or circle and enter the slab size. Switch between feet and metres at the top.
- Enter the thickness. Most patios are 4 in (10 cm); driveways 4–6 in (10–15 cm).
- Keep at least 5% extra. Use 10% if the ground is uneven or the forms are long.
- Optional: add the ready-mix price to estimate cost, and the truck size to see how many loads you need.
Concrete volume formula
Volume (ft³) = Length (ft) × Width (ft) × Thickness (ft) Cubic yards = Cubic feet ÷ 27 Cubic metres = Length (m) × Width (m) × Thickness (m)
Thickness is usually given in inches, so divide by 12 first: 4 in = 0.333 ft, 6 in = 0.5 ft. For a round slab, area = π × (diameter ÷ 2)².
Worked example
A 20 × 10 ft patio, 4 inches thick, with 5% extra, at $150 per cubic yard:
- 20 × 10 × 0.3333 = 66.67 ft³
- 66.67 ÷ 27 = 2.47 yd³
- +5% = 2.59 yd³ → order 2.75 yd³ (most suppliers sell in quarter yards)
- Cost ≈ 2.59 × $150 = $389 plus delivery
Coverage per cubic yard
| Thickness | Area per yd³ | Area per m³ |
|---|---|---|
| 3 in (7.5 cm) | 108 ft² | 13.3 m² |
| 4 in (10 cm) | 81 ft² | 10 m² |
| 5 in (12.5 cm) | 64.8 ft² | 8 m² |
| 6 in (15 cm) | 54 ft² | 6.7 m² |
| 8 in (20 cm) | 40.5 ft² | 5 m² |
Slab building tips
- Prepare the sub-base. 4–6 in (10–15 cm) of compacted crushed stone gives drainage and stops settlement cracks. Use our Gravel Calculator to size it.
- Reinforce. Wire mesh or rebar (or fibre-reinforced mix) holds cracks tight. Driveways usually need rebar.
- Plan control joints every 8–12 ft (2.5–3.5 m), and at about a quarter of the slab depth, so cracks form in straight lines.
- Check truck access. A full mixer weighs over 30 tonnes; it can crack driveways and lawns. Budget for a pump or wheelbarrows if it cannot reach.
- Cure it. Keep the slab damp or covered for a week; don't drive on it for 7–10 days.
Choosing the right thickness and mix
| Project | Typical thickness | Typical strength |
|---|---|---|
| Walkway, patio, shed base | 4 in (10 cm) | 2,500–3,000 psi (C20) |
| Car driveway | 4–5 in (10–12.5 cm) | 3,000–4,000 psi (C25–C30) |
| Garage floor, RV pad | 5–6 in (12.5–15 cm) | 3,500–4,000 psi (C25–C30) |
| Heavy trucks, workshop machinery | 6–8 in (15–20 cm) | 4,000+ psi (C30+) |
These are common starting points, not structural design. In cold climates, ask for air-entrained concrete for anything outdoors, as it resists freeze–thaw damage and de-icing salts. When you call the supplier, give them the cubic yards (or m³), the strength, the slump you want and whether you need a pump.