How thick should a concrete slab be?
| Use | Typical thickness |
|---|---|
| Walkways and paths | 4 in |
| Patios and shed bases | 4 in |
| Hot tub pads | 4 to 6 in |
| Driveways | 5 to 6 in |
| Garage floors | 4 to 6 in |
Concrete slab sizes in cubic yards
| Slab size | yd³ at 4 in | yd³ at 6 in | 80 lb bags at 4 in |
|---|---|---|---|
| 10 × 10 ft | 1.23 | 1.85 | 56 |
| 12 × 12 ft | 1.78 | 2.67 | 80 |
| 12 × 20 ft | 2.96 | 4.44 | 134 |
| 16 × 20 ft | 3.95 | 5.93 | 178 |
| 20 × 20 ft | 4.94 | 7.41 | 223 |
| 24 × 24 ft | 7.11 | 10.67 | 320 |
| 30 × 30 ft | 11.11 | 16.67 | 500 |
How to calculate a concrete slab
For an L-shaped patio, split it into two rectangles, work out each one, and add them together. You can set "How many slabs" to repeat identical slabs.
Slab prep checklist
- Dig out topsoil and lay about 4 in of compacted gravel for drainage and support.
- Set forms level, with a slight slope away from the house so water runs off.
- Add wire mesh or rebar for driveways and anything carrying heavy loads.
- Plan control joints every 8 to 10 ft so cracks form along neat lines.
How thick should a concrete slab be?
| Use | Typical thickness |
|---|---|
| Paths | 10 cm |
| Patios and shed bases | 10 cm |
| Hot tub pads | 10 to 15 cm |
| Driveways | 12.5 to 15 cm |
| Garage floors | 10 to 15 cm |
Concrete slab sizes in cubic metres
| Slab size | m³ at 10 cm | m³ at 15 cm | 25 kg bags at 10 cm |
|---|---|---|---|
| 3 × 3 m | 0.90 | 1.35 | 77 |
| 4 × 4 m | 1.60 | 2.40 | 137 |
| 4 × 6 m | 2.40 | 3.60 | 206 |
| 5 × 6 m | 3.00 | 4.50 | 257 |
| 6 × 6 m | 3.60 | 5.40 | 308 |
| 7 × 7 m | 4.90 | 7.35 | 419 |
| 9 × 9 m | 8.10 | 12.15 | 692 |
How to calculate a concrete slab
For an L-shaped patio, split it into two rectangles, work out each one, and add them together. You can set "How many slabs" to repeat identical slabs.
Slab prep checklist
- Dig out topsoil and lay about 10 cm of compacted hardcore or gravel for drainage and support.
- Set forms level, with a slight slope away from the house so water runs off.
- Add steel mesh or rebar for driveways and anything carrying heavy loads.
- Plan control joints every 2.5 to 3 m so cracks form along neat lines.
Common questions
How many yards of concrete do I need for a 20×20 slab?
At 4 inches thick, 4.94 cubic yards, or about 5.43 with 10% extra. At 6 inches it's 7.41 cubic yards.
How much concrete do I need for a 6 × 6 m slab?
At 10 cm thick, 3.6 m³, or about 3.96 m³ with 10% extra. At 15 cm it's 5.4 m³.
How thick should a concrete patio be?
4 inches is standard for a patio with foot traffic and furniture. Go to 5 or 6 inches where a hot tub or vehicle will sit.
How thick should a concrete patio be?
10 cm is standard for a patio with foot traffic and furniture. Go to 12.5 or 15 cm where a hot tub or vehicle will sit.
How much concrete for a 12×12 shed base?
At 4 inches thick, 1.78 cubic yards, or 80 bags of 80 lb before extra for waste.
How much concrete for a 3 × 4 m shed base?
At 10 cm thick, 1.2 m³, or 103 bags of 25 kg before extra for waste.
Do I need gravel under a concrete slab?
For most outdoor slabs, yes. A compacted gravel base helps drainage and stops the slab settling unevenly.
Do I need hardcore under a concrete slab?
For most outdoor slabs, yes. A compacted hardcore or gravel base helps drainage and stops the slab settling unevenly.