Thickness is the single biggest driver of how much concrete a slab needs and how much load it can carry. Too thin and it cracks under use. Too thick and you waste money on concrete the job never needed. The right number depends entirely on what will sit or drive on it.
Recommended thicknesses
| Use | Thickness |
|---|---|
| Patios, walkways, shed floors | 4 inches (10 cm) |
| Car driveways | 4 to 5 inches |
| Garage floors | 5 to 6 inches (13 to 15 cm) |
| Heavy vehicles, RV pads | 6 inches or more |
The base matters as much as the slab
A slab is only as good as what supports it. A well compacted base of about 4 inches of gravel spreads load and drains water away from the underside. Skimping on the base, or pouring on soft or uncompacted ground, leads to settling and cracking no matter how thick the concrete is. Compact the subgrade and the gravel before you pour.
Reinforcement
Many slabs use wire mesh or rebar set in the middle of the pour. These add tensile strength and control cracking but do not change the concrete volume much. A simple patio may be fine without it, while a driveway taking vehicle loads usually benefits from mesh or a grid of rebar. Fibre reinforcement mixed into the concrete is another option for crack control.
How thickness changes the concrete
Volume scales directly with thickness. Going from 4 to 6 inches adds 50 percent more concrete. A 20 by 20 slab is about 4.9 cubic yards at 4 inches but 7.4 at 6 inches, so the thickness decision has a real cost attached. Always measure the realistic thickness, including any thickened edges, which are common around the perimeter of a slab to carry edge loads.
Control joints
Concrete cracks. The question is where. Control joints, tooled or cut into the slab at regular spacing, give cracks a planned place to form so the surface stays neat. A common rule is joint spacing in feet no more than two to three times the slab thickness in inches, so a 4 inch slab gets joints every 8 to 12 feet. Cut or tool them in early, before the slab cures hard.
When to call a professional
For structural slabs, anything supporting a building, or work governed by local code, follow an engineer or your local building requirements rather than a rule of thumb. The numbers here are for typical flatwork like patios and driveways, not foundations.