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 |
Common practice for home projects. Your local building code may require more.
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 |
Before the extra for waste. Add 5 to 10% when you order.
How to calculate a concrete slab
Length (ft) × Width (ft) × Thickness (in) ÷ 12 = cubic feet
Cubic feet ÷ 27 = cubic yards
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.
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 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 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.
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.