Short answer: A concrete slab should be at least 4 inches thick for a patio, walkway or shed base. Driveways and garage floors are typically 5 to 6 inches, and a slab that carries heavy trucks or an RV is 6 inches or more.
| Project | Typical thickness | Cubic yards per 100 sq ft | 80 lb bags per 100 sq ft |
|---|---|---|---|
| Walkway or sidewalk | 4 in | 1.23 | 56 |
| Patio | 4 in | 1.23 | 56 |
| Shed or small building base | 4 in | 1.23 | 56 |
| Driveway for cars | 5 in | 1.54 | 70 |
| Garage floor | 5 in | 1.54 | 70 |
| Heavy truck or RV pad | 6 in | 1.85 | 84 |
Thickness is only part of a good slab
A slab lasts when the ground under it is firm. Most guides call for about 4 inches of compacted gravel under the concrete, which helps drainage and keeps the slab from settling unevenly. Driveways and garage floors are usually reinforced with welded wire mesh or rebar.
Control joints are cut or tooled into the slab so it cracks along planned lines. A common rule of thumb places them at spacing in feet equal to two to three times the thickness in inches, so about 8 to 12 feet for a 4 inch slab.
Check the depth of your forms
A board sold as a 2x4 is really 3.5 inches tall, so forms made from 2x4s hold a slab only 3.5 inches thick. For a 4 inch or thicker pour, use 2x6 forms, which are 5.5 inches tall, and set the gravel base to match. A slab thinner than planned is more likely to crack.
Local codes set the minimum for structural slabs, house foundations and footings, which are thicker at the edges. Check your local requirements and talk to the inspector before pouring anything that holds up a building.
Worked example
A driveway pad 10 ft by 20 ft has 200 square feet. At 4 inches thick it needs 2.47 cubic yards, and at 5 inches it needs 3.09, so the extra inch costs 0.62 cubic yards, or about 28 more bags of 80 lb mix.
Use the Concrete CalculatorEnter your own measurements and get an exact amount.
Frequently asked questions
Is a 4 inch concrete slab thick enough for a driveway?
It can carry cars, but many builders pour 5 or 6 inches for a driveway because the extra thickness resists cracking. For delivery trucks or an RV, use 6 inches.
How thick should a concrete garage floor be?
Typically 4 to 6 inches, and 5 or 6 inches if you park heavy vehicles or store heavy equipment.
How thick should a concrete patio be?
Four inches is the standard for a patio that carries furniture and people.
Does a thicker slab need rebar?
A thicker slab is stronger, but reinforcement still helps control cracking, especially on driveways and over poor soil. Ask a local professional about your ground.
More quick answers
- How Many Bags of Concrete Do I Need?
- How Many Yards Does a Concrete Truck Hold?
- How Many Bags of Concrete Are on a Pallet?
- How Many Square Feet Does a Yard of Concrete Cover?
- How Many Cubic Feet in a Bag of Concrete?
- How Much Does a Yard of Concrete Weigh?
Related calculators
Figures are typical planning values. Material sizes, densities and local requirements vary, so confirm with your supplier before ordering.
Last updated October 11, 2026.