How much concrete for a 4x4 m slab? (3x3 to 6x8 m)
A 4 x 4 m slab poured 100 mm thick is 1.6 m³ of concrete; with 10% waste that is 1.76 m³, which means 147 bags of 25 kg (3,675 kg of dry mix) or a ready-mix order of 2 m³. At 150 mm it is 2.4 m³ and 220 bags. Below about 1 m³ bags are realistic; above it you order a truck. The tables cover 3 x 3 to 6 x 8 m at 100, 125 and 150 mm.
Slabs at 100 mm
Bag yields as printed on typical bags: about 12 litres of concrete per 25 kg bag and 10 litres per 20 kg bag, with 10% waste for uneven ground and formwork. Ready-mix is rounded up to the next quarter cubic metre.
| Slab | Concrete (m³) | 25 kg bags | 20 kg bags | Dry mix (kg, 25 kg bags) | Ready-mix order (m³) |
|---|---|---|---|---|---|
| 3 x 3 m | 0.9 | 83 | 99 | 2,075 | 1 |
| 4 x 4 m | 1.6 | 147 | 176 | 3,675 | 2 |
| 4 x 6 m | 2.4 | 220 | 264 | 5,500 | 2.75 |
| 5 x 5 m | 2.5 | 230 | 275 | 5,750 | 2.75 |
| 6 x 6 m | 3.6 | 330 | 396 | 8,250 | 4 |
| 6 x 8 m | 4.8 | 440 | 528 | 11,000 | 5.5 |
Slabs at 125 mm
| Slab | Concrete (m³) | 25 kg bags | 20 kg bags | Ready-mix order (m³) |
|---|---|---|---|---|
| 3 x 3 m | 1.13 | 104 | 124 | 1.25 |
| 4 x 4 m | 2 | 184 | 220 | 2.25 |
| 4 x 6 m | 3 | 275 | 330 | 3.5 |
| 5 x 5 m | 3.13 | 287 | 344 | 3.5 |
| 6 x 6 m | 4.5 | 413 | 495 | 5 |
| 6 x 8 m | 6 | 550 | 660 | 6.75 |
Slabs at 150 mm for garages and driveways
| Slab | Concrete (m³) | 25 kg bags | 20 kg bags | Ready-mix order (m³) |
|---|---|---|---|---|
| 3 x 3 m | 1.35 | 124 | 149 | 1.5 |
| 4 x 4 m | 2.4 | 220 | 264 | 2.75 |
| 4 x 6 m | 3.6 | 330 | 396 | 4 |
| 5 x 5 m | 3.75 | 344 | 413 | 4.25 |
| 6 x 6 m | 5.4 | 495 | 594 | 6 |
| 6 x 8 m | 7.2 | 660 | 792 | 8 |
How the numbers are worked out
Length x width x thickness in metres: 4 x 4 x 0.1 = 1.6 m³. Multiply by 1.10 for waste, then divide by the bag yield and round up, or round up to the supplier's quarter cubic metre. The concrete calculator does any rectangle or circle in metric or imperial. The imperial slabs are in the 10x10, 12x12 and 4x4 ft guides.
Bags or a truck
A 25 kg bag makes about 12 litres, so a cubic metre is roughly 84 bags and 2,100 kg of dry mix to lift twice. Up to about 1 m³ that is a long day with a mixer; beyond it, ready-mix is cheaper per cubic metre and arrives in one consistent batch. Ask the plant for its minimum load and part-load charge, and read the bagged vs ready-mix break-even before deciding.
Before you pour
- Dig out the topsoil, lay 100 to 150 mm of compacted hardcore and a damp-proof membrane under a floor that will be enclosed; the shed base guide works out the sub-base.
- Use A142 mesh on anything over 3 x 3 m or thicker than 100 mm, held at mid-depth on spacers.
- Set the formwork level with a slight fall (about 1 in 80) away from buildings, and pour the whole slab in one session.
- Cure for at least three days under polythene or by keeping it damp; keep cars off for a week.
Frequently asked questions
- How many 25 kg bags of concrete for a 4x4 m slab?
- 147 bags at 100 mm thick with 10% waste (1.6 m³ of concrete), 184 at 125 mm and 220 at 150 mm. In 20 kg bags: 176, 220 and 264. That is 3,675 kg of dry mix at 100 mm, so most people order 2 m³ of ready-mix instead.
- How many cubic metres of concrete for a 4x4 slab?
- 1.6 m³ at 100 mm (4 x 4 x 0.1), 2 m³ at 125 mm and 2.4 m³ at 150 mm, before waste. Add 10% for uneven ground and spillage and round up to the supplier's step, usually a quarter cubic metre: order 2 m³ for 100 mm.
- How much concrete for a 3x3 m shed base?
- 0.9 m³ at 100 mm, which is 83 bags of 25 kg or 99 of 20 kg with 10% waste. A 3 x 3 m base is the largest slab most people mix from bags: 2,075 kg of dry mix is a full day for two with a mixer.
- How much concrete for a 6x6 m garage or double slab?
- 3.6 m³ at 100 mm and 5.4 m³ at 150 mm, before waste; order 4 or 6 m³. In bags that would be 330 of 25 kg at 100 mm, which is not a bag job. A 6 x 8 m slab at 150 mm is 8 m³, a normal small ready-mix load.
- How thick should a metric slab be?
- 100 mm for a shed base, patio, path or hot tub pad on well compacted hardcore; 125 to 150 mm with mesh for a garage, workshop floor or anything a car parks on. Thinner than 75 mm cracks. Always on 100 to 150 mm of compacted sub-base, never straight onto soil.
- Should I use bags or ready-mix for a 4x4 m slab?
- Ready-mix, unless access is impossible. 147 bags of 25 kg is 3,675 kg to carry and mix, and the slab should be poured in one go. Price 2 m³ delivered (small loads carry a part-load charge) against the bags; the truck usually wins on anything over about 1 m³.
More guides
- How many bags of concrete for a 10x10 slab?
Bags of 80, 60 and 40 lb concrete for a 10 x 10 ft slab at 4, 5 and 6 inches with 10% waste, plus the metric 3 x 3 m version in 25 kg and 20 kg bags.
- How many bags of concrete per fence post?
Bags of 80, 60 and 50 lb concrete per 4x4 or 6x6 fence post for 8, 10 and 12 inch holes at 24, 30 and 36 inches deep, plus the metric 100 mm post in 25 kg bags.
- How many bags of concrete in a cubic yard?
80, 60, 50 and 40 lb bags per cubic yard and per cubic foot, 25 kg and 20 kg bags per cubic metre, what a yard of bags weighs, and when a ready-mix truck makes sense.
- How much concrete for a 12x12 slab? (plus 16x16, 20x20, 24x24)
Cubic yards and bags of concrete for 12x12, 16x16, 20x20 and 24x24 ft slabs at 4 and 6 inches with 10% waste, the 4 x 4 m metric slab, and the point where ready-mix wins.
Last updated 2026-09-13. Figures are planning estimates from typical product data: check the yield, size or coverage printed on what you buy and round up.