How it works
Concrete is ordered by volume: cubic yards in the US, cubic metres almost everywhere else. For small jobs you buy pre-mixed bags. The calculator finds the slab volume, adds extra for spillage and uneven ground, and converts it to bags.
volume = length × width × thickness × number of slabs × (1 + waste %) 1 yd³ = 27 ft³ = 0.7646 m³ · bags = volume in ft³ ÷ bag yield (80 lb ≈ 0.60 ft³, 60 lb ≈ 0.45 ft³, 40 lb ≈ 0.30 ft³)Worked examples. A 4 m × 3 m slab, 10 cm thick, with 10% waste = 1.2 × 1.1 = 1.32 m³ (1.73 yd³), or 78 bags of 80 lb. A 10 × 10 ft patio, 4 inches thick, with no waste = 33.33 ft³ = 1.23 yd³ = 56 bags of 80 lb.
Typical thickness
| Job | Thickness |
|---|---|
| Paths, shed bases | 3–4 in (7.5–10 cm) |
| Patios, house slabs | 4 in (10 cm) |
| Driveways for cars | 5–6 in (12.5–15 cm) |
| Heavy vehicles | 6–8 in (15–20 cm) with reinforcement |
Bags or ready-mix truck?
- Up to about 1 cubic yard (0.75 m³), bags are usually practical. Above that, a ready-mix delivery is faster and gives more even concrete.
- Ready-mix suppliers often have a minimum order and charge for short loads, so round up to the next quarter or half yard.
- Bag yields vary by brand; check the yield printed on the bag.
Waste allowance
Add 5–10% for spillage, form bulging and uneven sub-base. Running short in the middle of a pour leaves a cold joint, so it is better to have a little left over.
Need the slab area first? Use the square footage calculator. Convert measurements with feet to meters or cm to inches.