Quick answer: A 4 m × 3 m slab 10 cm thick needs 4 × 3 × 0.10 = 1.2 m³ of concrete; with 5% extra that is 1.26 m³, or 68 bags of 40 kg premix.
How it's calculated
Slab or footing: volume = length × width × thickness, with all three in metres (10 cm = 0.10 m). Round column: volume = π × radius² × height. Multiply by the number of identical pieces.
Add 5–10% for spillage, uneven ground and formwork that bows. Normal-weight concrete weighs about 2,400 kg per m³, so 1 m³ is roughly 2.4 tonnes. Some calculators give the weight of dry bagged mix instead (about 2,130 kg per m³), so their weight is lower for the same volume.
Premixed bags yield about 0.60 ft³ (17 L) per 80 lb (36.3 kg) bag, which is about 18.7 L for a 40 kg bag. Bags needed = total volume ÷ yield per bag, rounded up.
Worked examples
- Slab 4 m × 3 m × 10 cm, 5% waste: 1.2 m³ × 1.05 = 1.26 m³ → 68 bags of 40 kg
- Round column 30 cm × 3 m, no waste: π × 0.15² × 3 = 0.212 m³
- Patio 12 ft × 10 ft × 4 in: 12 × 10 × 0.333 = 40 ft³ = 1.48 yd³
Premixed bag yields
Approximate yield of packaged concrete mix (0.0075 ft³ per lb).
| Bag | Yield | Bags per m³ |
|---|---|---|
| 20 kg | 9.4 L | 107 |
| 25 kg | 11.7 L | 86 |
| 40 kg | 18.7 L | 54 |
| 50 kg | 23.4 L | 43 |
| 60 lb | 0.45 ft³ (12.7 L) | 79 |
| 80 lb | 0.60 ft³ (17 L) | 59 |
Frequently asked questions
How thick should a slab be?
Paths and patios are commonly 10 cm (4 in); driveways and garage floors usually 12–15 cm. Structural slabs and footings follow the engineer's drawings.
How much extra concrete should I order?
5% is usual for a level, well-formed slab; use 10% for uneven ground or many small pours.
How many bags make one cubic metre?
About 54 bags of 40 kg premix, or 59 bags of 80 lb. The exact yield is printed on the bag.
How heavy is one cubic metre of concrete?
About 2,400 kg for normal-weight concrete; reinforced concrete is often taken as 2,500 kg.
How much concrete do I need for a 4 × 3 m slab?
1.2 m³ at 10 cm thick. Order 1.26 m³ to leave 5% for spills and uneven ground.
Should I use ready-mix or bags?
Bags are fine for fence posts or a small pad. Once you're past about 1 m³ (that's around 54 bags of 40 kg), a ready-mix truck is usually cheaper and far less work.