How Thick Should a Concrete Slab Be? Thickness by Use and Cost
A concrete patio, walkway or shed slab is 4 inches thick, and a driveway or garage floor is 4 to 6 inches. Going from 4 to 6 inches adds half as much concrete again: a 10 × 10 ft slab takes 1.36 cubic yards at 4 inches and 2.04 at 6 inches (with 10% waste). The concrete calculator gives the volume and the bags for your own slab, and the whole shopping list is in the concrete slab materials calculator.
Slab thickness by use
| Use | Typical thickness | Notes |
|---|---|---|
| Patio, walkway or sidewalk | 4 in | Foot traffic and furniture |
| Shed slab or small outbuilding | 4 in | Light loads: use a thicker edge for heavier sheds |
| Residential driveway | 4 to 5 in | Cars and light trucks; 6 in if heavy trucks or an RV will use it |
| Garage floor | 4 to 6 in | 4 in for cars; 5 to 6 in for heavy vehicles, a lift or a workshop |
| Basement floor | 3½ to 4 in | Over a vapor barrier and gravel |
| Steps and landings | 4 to 6 in | Thicker for a landing that carries a rail |
These are typical residential figures. Your building code, the soil and the loads decide the thickness for your project, so check with your local building department, and ask an engineer about anything that carries a structure.
How much concrete for each thickness
Cubic yards for a ready-mix order, and 80 lb bags in brackets, with 10% waste:
| Slab | 4 in | 5 in | 6 in |
|---|---|---|---|
| 10 × 10 ft | 1.36 cu yd (62 bags) | 1.70 cu yd (77 bags) | 2.04 cu yd (92 bags) |
| 12 × 12 ft | 1.96 cu yd (88 bags) | 2.44 cu yd (110 bags) | 2.93 cu yd (132 bags) |
| 12 × 20 ft | 3.26 cu yd (147 bags) | 4.07 cu yd (184 bags) | 4.89 cu yd (220 bags) |
| 20 × 20 ft | 5.43 cu yd (245 bags) | 6.79 cu yd (306 bags) | 8.15 cu yd (367 bags) |
| 24 × 24 ft | 7.82 cu yd (352 bags) | 9.78 cu yd (440 bags) | 11.73 cu yd (528 bags) |
| 24 × 30 ft | 9.78 cu yd (440 bags) | 12.22 cu yd (550 bags) | 14.67 cu yd (660 bags) |
What goes under and in the slab
- A compacted sub-base of gravel, about 4 inches thick, drains water and gives an even bed. Remove topsoil and soft ground first.
- Reinforcement. Wire mesh or rebar on chairs holds a cracked slab together. Driveways and garage floors usually have it.
- Control joints. Cut or tool a groove a quarter of the depth of the slab, so that cracks form in the joint:
| Slab thickness | Joint spacing (both directions) |
|---|---|
| 4 in | 8 to 12 ft |
| 5 in | 10 to 15 ft |
| 6 in | 12 to 18 ft |
- Curing. Keep the slab damp, or cover it, for the first seven days. Concrete reaches most of its strength in a month.
- Slope. Give an outdoor slab a fall of about 1/8 to 1/4 inch per foot so that water drains away.
The cost of extra thickness
Thickness is the cheapest way to make a slab stronger, but the concrete is a large part of the cost, so a 6 inch slab uses 50% more than a 4 inch one. For a driveway, find the thickness your loads need and no more. See the whole list in the materials list for a concrete driveway and the bags of concrete guide.
Frequently asked questions
How thick should a concrete patio be?
Four inches is the standard thickness for a patio, a walkway or a sidewalk, on a compacted gravel base about four inches thick.
How thick should a concrete driveway be?
At least four inches for cars and light trucks, and five or six inches if heavy trucks, a motorhome or a trailer will use it. Check local code, since some places require more.
How much more concrete does a 6 inch slab need than a 4 inch slab?
Half as much again: a 10 × 10 ft slab takes 1.36 cubic yards at 4 inches and 2.04 at 6 inches, with 10% waste.
How far apart should control joints be?
About two to three times the slab thickness in inches, in feet: a 4 inch slab needs a joint every 8 to 12 feet, in both directions, cut to a quarter of the depth.
Does a concrete slab need rebar or wire mesh?
Reinforcement helps control cracks and hold a cracked slab together, and is usual for driveways and garage floors. Ask your local building department what it requires.
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Estimates only. Quantities depend on your materials, site and supplier. Confirm important numbers before you order, and follow local building codes for anything structural.