How Thick Should a Concrete Slab Be? (Patio, Driveway, Floor)
Short answer:
- 4 inches for patios, walkways and driveways used by cars.
- 5 to 6 inches for driveways that see light to medium trucks.
- 7 inches or more for heavy trucks, which should be designed by an engineer.
Concrete floors inside a house must be at least 3½ inches under the building code. Every slab sits on a compacted gravel base, not bare soil.
Once you know the thickness, the concrete calculator works out cubic yards and bags.
Thickness by use
| Slab | Thickness | Source |
|---|---|---|
| House floor (slab on ground) | 3½ in minimum | 2021 IRC R506.1 |
| Patio, walkway, small pad | 4 in, on 3 in of gravel | Quikrete slab instructions |
| Driveway, cars and light trucks | 4 in | Michigan Concrete Association (“light use”) |
| Driveway, light-to-medium trucks | 5 to 6 in | Michigan Concrete Association (“medium use”) |
| Heavy commercial trucks | 7 in or thicker, designed case by case | Michigan Concrete Association (“heavy use”) |
Things that may call for a thicker slab: soft or clay soil, frequent heavy loads (an RV, a boat trailer, a delivery truck), or a heavy point load such as a hot tub (check the maker’s requirements). Ask your local building department about anything structural.
The base under the slab
- For a patio or walk, Quikrete’s instructions are to dig about 7 inches deep: 3 inches of gravel, compacted, plus 4 inches of concrete.
- Under a house floor that’s below grade, the IRC requires a 4 inch base of clean graded sand, gravel or crushed stone (R506.2.2), plus a 10-mil vapor retarder (R506.2.3).
- Driveways, walks and patios don’t need the vapor retarder.
How much concrete each thickness takes
| Slab | Thickness | Cubic yards | 80 lb bags |
|---|---|---|---|
| 10 × 10 ft | 3½ in | 1.08 | 49 |
| 10 × 10 ft | 4 in | 1.23 | 56 |
| 10 × 10 ft | 5 in | 1.54 | 70 |
| 10 × 10 ft | 6 in | 1.85 | 84 |
| 12 × 20 ft | 4 in | 2.96 | 134 |
| 12 × 20 ft | 5 in | 3.70 | 167 |
| 12 × 20 ft | 6 in | 4.44 | 200 |
These are exact volumes, with no extra included. Add about 10% for uneven ground and spillage. Going from 4 to 5 inches adds 25% more concrete. Bag counts use Quikrete’s yield of 0.60 cu ft per 80 lb bag.
Control joints
Cut or tool control joints so the slab cracks along a straight line instead of randomly. Quikrete says the joints should go at least a quarter of the slab’s depth, which is 1 inch in a 4 inch slab.
Common questions
Is 4 inches of concrete enough for a driveway?
For cars and light trucks, yes. The Michigan Concrete Association puts 4 inches at “light use”. If trucks, an RV or heavy deliveries will use it, go to 5 to 6 inches.
How thick should a shed or garage slab be?
A garage floor counts as a slab-on-ground floor, so the 3½ inch IRC minimum applies. If you’ll park a car on it, 4 inches matches the Michigan Concrete Association’s “light use” driveway figure. Garages don’t need the vapor retarder; check your local code.
Can a concrete slab be 2 or 3 inches thick?
Thin toppings over existing concrete exist, but a standalone slab on the ground should be at least 3½ to 4 inches.
Sources
- International Code Council: 2021 IRC section R506: slab thickness, base course and vapor retarder rules.
- Michigan Concrete Association: recommended concrete thicknesses: light, medium and heavy use.
- Quikrete: pouring and finishing concrete slabs: 3 in gravel plus 4 in concrete; joints at least ¼ of the depth.
- Quikrete Concrete Mix (No. 1101) data sheet: 80 lb bag yields 0.60 cu ft.