Common slab thicknesses
| Project | Typical thickness |
|---|---|
| Garden path or walkway | 4 in (100 mm) |
| Patio | 4 in (100 mm) |
| Shed or small garden building | 4 in (100 mm) |
| Hot tub pad | 4 to 6 in (100 to 150 mm), check the tub maker's requirements |
| Residential driveway, cars | 4 to 6 in (100 to 150 mm); 5 in or more for heavier vehicles |
| Heavy vehicles, RVs | Engineered design |
These figures are common practice, not a design. Soil type, frost depth, reinforcement and local building codes all affect the right answer. For garages, house slabs and anything that carries a building, follow your local code or an engineer.
What else matters as much as thickness
The base
A slab is only as good as what is under it. Most outdoor slabs sit on a compacted layer of crushed stone or gravel. A soft or uneven base lets the slab settle and crack, however thick it is.
Reinforcement
Steel mesh or rebar does not stop concrete from cracking, but it holds cracks tight so the slab stays level. Driveways and larger slabs commonly use it. Some contractors use fibre-reinforced mixes for smaller slabs.
Control joints
Concrete shrinks as it cures. Joints cut or tooled into the surface give it a planned place to crack. A common rule of thumb: in feet, space joints no more than two to three times the slab thickness in inches. That is about 8 to 12 ft for a 4 in slab.
How thickness changes your order
Volume rises directly with thickness. Going from 4 to 6 inches adds 50 percent more concrete.
| 12 × 20 ft shed base | Volume | With 10% extra |
|---|---|---|
| 4 in thick | 2.96 yd³ | 3.26 yd³ |
| 5 in thick | 3.70 yd³ | 4.07 yd³ |
| 6 in thick | 4.44 yd³ | 4.89 yd³ |
At this size, the extra 2 inches adds about 1.6 cubic yards to the order, often the difference between a small part load and a much larger one. Ask your ready-mix supplier about their minimum load and short-load fee before you settle on a thickness.
Use the concrete calculator to get the volume and bag count for your own size.
Thickened edges
Many patios and shed bases are built with edges thicker than the middle, because the edges carry more load and are more exposed. If your slab has thickened edges, calculate the main slab and the edge strip separately and add them.
Frequently asked questions
Is 3 inches of concrete enough for a patio?
Many guides and codes treat 4 inches as the practical minimum for a patio or walkway. Thinner slabs crack more easily.
How thick should a shed base be?
A 4 inch slab on a compacted gravel base is common for garden sheds. Larger buildings or workshops may need more, and may need permits.
Does a thicker slab need more reinforcement?
Reinforcement depends on the load and slab size as well as thickness. For driveways and larger slabs, follow local practice or an engineer.
Do the math for your own project.
Open the concrete calculator