Slab thickness is the first question on any concrete project, and getting it wrong is expensive — too thin and it cracks, too thick and you waste material. The right number depends almost entirely on how much weight the slab will carry.
Recommended slab thickness by project
| Project | Typical thickness | Reinforcement |
|---|---|---|
| Walkway / garden path | 4 inches | Optional wire mesh |
| Patio | 4 inches | Wire mesh or #3 rebar grid |
| Shed floor | 4 inches | Wire mesh |
| Car driveway / garage | 5–6 inches | #3–#4 rebar grid |
| RV pad / heavy workshop | 6+ inches | #4 rebar grid |
Why the base matters as much as thickness
A slab is only as strong as what's under it. Pour over 4 to 6 inches of compacted gravel or crushed stone to provide drainage and a stable, even bearing surface. Skipping the base is a common reason slabs crack, even when they're thick enough.
To order the right amount of concrete, measure your slab's area and thickness and run it through our cubic yard calculator, which converts the dimensions into cubic yards and includes a small overage so you don't come up short mid-pour.
Do I need rebar or wire mesh?
Reinforcement doesn't stop concrete from cracking, but it holds any cracks tightly together so the slab stays structurally sound. Thin, lightly loaded slabs can use welded wire mesh; driveways and anything carrying vehicles should use a rebar grid. Our rebar calculator works out how many bars and what total length you need for a given grid spacing.
How much concrete will I need?
Concrete is sold by the cubic yard. Multiply the slab's length by its width by its thickness (in feet) to get cubic feet, then divide by 27. Rather than doing that by hand, enter your dimensions into the cubic yard calculator and it returns the volume, plus an estimate you can take straight to the supplier.
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Frequently Asked Questions
How thick should a concrete slab be?
Four inches is standard for patios, walkways, and shed floors, while driveways and garages need 5 to 6 inches, and heavy loads like RV pads need 6 inches or more.
How thick should a concrete driveway be?
A residential concrete driveway should be at least 5 inches thick, or 6 inches if it will carry heavy vehicles, over a compacted gravel base with a rebar grid.
Do I need rebar in a 4-inch slab?
A lightly loaded 4-inch slab can use welded wire mesh instead of rebar, but any slab carrying vehicles should have a rebar grid to hold cracks together.
How much gravel goes under a concrete slab?
Most slabs sit on 4 to 6 inches of compacted gravel or crushed stone, which provides drainage and a stable base that helps prevent cracking.