How much concrete do I need?
Last reviewed 1 July 2026
Short answer
Volume is length × width × thickness, with the thickness converted to the same unit as the other two. A 3 m × 2.4 m slab at 100 mm is 0.72 m³. That is about 61 bags of 25 kg ready-to-use concrete, or a 0.8 m³ ready-mix order once waste is allowed — and it weighs about 1.7 tonnes.
Work it out for your project with the concrete slab calculator →
Getting the units right
The single most common mistake is mixing units: entering the thickness in millimetres while the length and width are in metres. A 100 mm slab is 0.1 m, so a 3 × 2.4 m slab is 3 × 2.4 × 0.1 = 0.72 m³, not 720 m³.
In imperial, convert inches to feet by dividing by 12. A 10 ft × 8 ft slab at 4 in is 10 × 8 × 0.3333 = 26.7 ft³, which is 0.99 cubic yards.
volume = length × width × thickness (all in the same unit)
Volumes for common jobs
| Job | Size | Thickness | Volume (m³) | Volume (yd³) |
|---|---|---|---|---|
| Shed base | 2.4 × 1.8 m (8 × 6 ft) | 100 mm (4 in) | 0.43 | 0.57 |
| Small patio slab | 3 × 2.4 m (10 × 8 ft) | 100 mm (4 in) | 0.72 | 0.94 |
| Garage floor | 6 × 3 m (20 × 10 ft) | 150 mm (6 in) | 2.70 | 3.53 |
| Single driveway | 10 × 3 m (33 × 10 ft) | 125 mm (5 in) | 3.75 | 4.90 |
| Fence post hole | 300 mm dia × 600 mm | — | 0.042 each | 0.055 each |
Footings and post holes
A strip footing is length × width × depth, the same as a slab, just deeper and narrower. A 12 m run of footing 600 mm wide and 200 mm deep is 1.44 m³.
A round post hole is π × radius² × depth. A 300 mm diameter hole 600 mm deep is π × 0.15² × 0.6 = 0.042 m³, or about 42 litres. Nine posts is 0.38 m³ — roughly 32 bags of 25 kg, which is more than most people expect from 'a few post holes'.
It is normal not to deduct the post itself. The small over-order covers spillage and the hole never being quite the shape you dug it.
Weight
Normal-weight concrete is about 2,400 kg/m³ wet. That 0.72 m³ patio slab is 1,730 kg — more than a tonne and a half of material to mix and move by hand.
Weight is the number that decides how the job is done. Under about 0.25 m³ is a comfortable afternoon with a mixer. Above 1 m³, mixing on site becomes a serious undertaking and ready-mix is almost always the right answer.
Important: Order the whole pour at once
How much extra to order
- 5–10% for a slab on a prepared base. Covers spillage and an uneven sub-base.
- 10–15% for footings. Trench sides are never as neat as the drawing, and over-dig is real.
- Ready-mix minimum. Most plants have a minimum load and charge a short-load fee below about 3–4 m³. Ask before you assume a 0.8 m³ order is possible.
- Never round down. Being 5% short on a pour is far more expensive than being 10% over.
Related questions
- How much does a cubic metre of concrete weigh?
- About 2,400 kg, or 2.4 tonnes, for normal-weight concrete. A cubic yard is about 4,050 lb, roughly 2 US tons.
- Is it cheaper to mix concrete myself?
- For very small volumes, yes, because there is no delivery minimum. Ready-mix usually becomes cheaper per cubic metre well before 1 m³, and it saves hours of labour. The crossover depends on your local delivery charges.
Safety notice: Material planning only