How Thick Should a Concrete Slab Be?
Four inches is standard for patios, walkways, shed floors, and anything that only carries foot traffic. Driveways and garage floors need 5 to 6 inches, and slabs carrying trucks, RVs, or heavy equipment need 6 inches or more.
Whatever the thickness, the compacted base under it matters just as much.
| Use | Thickness | Mix strength | Reinforcement |
|---|---|---|---|
| Walkway, garden path | 3–4 in | 3,000 psi | Optional mesh |
| Patio, shed floor | 4 in | 3,000 psi | 6×6 mesh or #3 rebar |
| Car driveway, garage floor | 4–5 in | 3,500–4,000 psi | #3–#4 rebar at 18 in |
| Truck / RV parking | 6 in | 4,000 psi | #4 rebar at 12–18 in |
| Hot tub pad | 6 in | 3,500–4,000 psi | #4 rebar at 12 in |
| Workshop, heavy equipment | 6–8 in | 4,000+ psi | Engineered |
General residential guidance. Local building code, soil conditions, and frost depth take priority.
What thickness actually buys you
Thickness mostly resists bending. A slab spans across soft spots in the subgrade, and a thicker slab can bridge a wider one before it cracks. Doubling from 4 to 6 inches adds 50% to the concrete cost but more than doubles the bending capacity.
What thickness does not fix is a bad base. A 6-inch slab on uncompacted fill will still settle and crack; a 4-inch slab on 6 inches of well-compacted gravel usually will not. Spend on the base first.
The base under the slab
- 4–6 inches of compacted crushed stone or gravel, in 2–3 inch lifts
- Compact each lift with a plate compactor — not just the top layer
- Vapour barrier under anything enclosed, heated, or later floored
- Slope the finished surface 1/8 to 1/4 inch per foot away from buildings
Thickened edges and footings
Slabs that carry a wall, a post, or vehicle wheel loads at the perimeter are usually thickened to 8–12 inches at the edge, tapering back into the field of the slab. Monolithic slabs for garages and outbuildings combine the footing and slab in one pour. Work out the extra volume with the concrete footing calculator and add it to your slab total.
Control joints
Concrete cracks — the aim is to decide where. Cut or tool control joints to a quarter of the slab depth (1 inch deep on a 4-inch slab), spaced in feet at roughly 2 to 3 times the thickness in inches:
- 4-inch slab — joints every 8 to 12 ft
- 5-inch slab — joints every 10 to 15 ft
- 6-inch slab — joints every 12 to 18 ft
Keep panels roughly square; long thin panels crack across the middle.
How thickness changes the order
For a 400 sq ft slab: 4.94 yards at 4 inches, 6.17 at 5 inches, 7.41 at 6 inches. Check the numbers for your dimensions with the concrete slab calculator before you commit — the jump from 4 to 6 inches can be the difference between one truck and two.
Estimating notes
- This is general guidance, not engineering advice. Structural slabs, expansive soils, and anything supporting a building need a design from a qualified engineer and a permit.
- In frost-prone regions, footings must reach below the local frost line regardless of slab thickness.
- Fibre reinforcement helps with shrinkage cracking but does not replace rebar for load-carrying slabs.