A concrete slab should be 4 inches thick for patios, walkways, shed floors and driveways that carry cars. Make it 5 to 6 inches where trucks, RVs or heavy equipment will park. For a house floor slab, the building code minimum is 3.5 inches.
Going thicker costs less than most people expect. On a 20x20 patio, the jump from 4 to 6 inches adds about 2.5 cubic yards of concrete, or roughly $458 at the current US average price.
Concrete slab thickness by use
Thickness is about what the slab has to carry. People and patio furniture barely load a slab at all. A parked truck puts its weight on four small patches, and that is what cracks a thin driveway. Most home projects land on 4 inches, and the exceptions are the ones with vehicles heavier than a car.
| Slab | Thickness | Notes |
|---|---|---|
| Sidewalk or garden path | 4 in | Foot traffic only |
| Patio | 4 in | Furniture, grills and people |
| Shed floor | 4 in | More if it will hold a tractor or heavy tools |
| Driveway, cars and SUVs | 4 in | Quikrete's figure for passenger vehicles |
| Driveway, trucks or an RV | 5 to 6 in | Delivery trucks, loaded pickups, campers |
| Garage floor | 4 in | 5 to 6 in for heavy trucks or a shop with heavy equipment |
| House floor, slab on ground | 3.5 in minimum | IRC R506.1; 4 in is the common pour |
| Hot tub pad | Per the spa maker | Follow the installation guide for your model |
Driveway advice online is split between 4 and 6 inches, and the split comes down to the vehicles. Quikrete puts it plainly in its driveway guide: 4 inches is enough when only passenger cars use it, and 5 to 6 inches is the call when trucks and other heavy vehicles will.
What the building code requires
The International Residential Code, which most US towns adopt in some form, covers the floor slab of a house. Section R506.1 sets the minimum thickness of a concrete slab-on-ground floor at 3.5 inches. Section R506.2.2 calls for a 4-inch base of gravel or crushed stone under it.
Patios, walks and driveways are a different story. The residential code says little about outdoor flatwork, so towns fill the gap with their own rules, most often for driveway aprons and public sidewalks. If any part of the pour touches the street or a public walk, ask the permit office for their spec before you order concrete. Poor soil or an unusual load, like a slab under a building or heavy machinery, is a job for an engineer, not a rule of thumb.
What each extra inch costs
Every inch is a quarter more concrete than a 4-inch slab. Here is what that means in yards and dollars per 100 square feet, at the current US ready-mix average of $185.38 per cubic yard (September 2026). The price is material only, before delivery fees and labor.
| Thickness | Yards per 100 sq ft | Concrete cost per 100 sq ft | 20x20 patio |
|---|---|---|---|
| 3.5 in | 1.08 | $200 | 4.32 yd, $801 |
| 4 in | 1.23 | $229 | 4.94 yd, $915 |
| 5 in | 1.54 | $286 | 6.17 yd, $1,144 |
| 6 in | 1.85 | $343 | 7.41 yd, $1,373 |
The concrete is rarely the expensive part. Labor, forms, base prep and finishing are priced by the square foot and barely change with an extra inch, so a thicker slab adds a modest share to the total. Ordering short costs more than ordering thick: a second truck for the last half yard usually comes with its own delivery and short-load fees. See the current number on the concrete price per yard page, or price the whole job installed with the Concrete Slab Cost Calculator.
Thickness, base and reinforcement
A thick slab on soft ground still cracks. The base under the concrete spreads the load, and a compacted layer of gravel or crushed stone is what most outdoor slabs sit on. Work out how much you need with the Gravel Calculator.
Rebar and wire mesh do a different job. They hold cracks tight once they form, which keeps the pieces level, but they do not let you pour thinner. Plan the thickness first, then add steel. The Rebar Calculator counts bars for a slab grid.
Small slabs: bags at each thickness
For a small pad, bags can make more sense than a truck. An 80 lb bag of concrete mix yields about 0.6 cubic feet, so the bag count climbs fast with thickness.
| Thickness | 10x10 pad | 80 lb bags |
|---|---|---|
| 3.5 in | 29.2 cu ft | 49 |
| 4 in | 33.3 cu ft | 56 |
| 5 in | 41.7 cu ft | 70 |
| 6 in | 50.0 cu ft | 84 |
Past about one cubic yard, which is 45 of those bags, mixing by hand turns into a long day and ready-mix usually wins on both price and labor.
Sources and how we checked
- International Residential Code, section R506 (Concrete floors on ground): 3.5-inch minimum slab and 4-inch base course. Read via the California Residential Code text of R506, which adopts the IRC wording. Your town may amend it.
- Quikrete, Concrete Driveways and Curbs: 4 inches for passenger cars, 5 to 6 inches for trucks and heavy vehicles.
- Quikrete, Concrete Mix data sheet: yield per bag, used for the bag counts.
- Price per yard: CalcRafter's concrete price index, the NRMCA national average carried forward monthly with the BLS ready-mix producer price index. Method on How We Calculate and Price.
Every yard, dollar and bag figure on this page is calculated from those inputs when the site is built, using the same formula as the Concrete Calculator, so the tables move when the price index does. Spot something wrong or out of date? Email soleman@calcrafter.com and the fix goes out in the next release.
Enter your own length, width and thickness in the Concrete Calculator to get yards and bags for your slab.
Skip the arithmetic
Enter your own dimensions and get the exact figure instead of working from averages.
Open the Concrete CalculatorFrequently asked questions
4 inches for most home projects: patios, walkways, shed floors and driveways that only see cars. Go to 5 or 6 inches where trucks, RVs or heavy equipment will sit or drive. For the floor slab of a house, the International Residential Code sets a 3.5-inch minimum (section R506.1), and most crews pour 4.
Not for anything that carries weight. Three inches is below the 3.5-inch code minimum for a house floor, and below the 4 inches that is standard for patios, walks and driveways. A thin slab saves very little concrete. On a 10x10 pad, going from 3 to 4 inches adds about a third of a yard.
4 inches if only cars and SUVs use it. Quikrete recommends 5 to 6 inches when trucks or other heavy vehicles will drive on it, which includes delivery trucks, loaded pickups and RVs. Many towns also set their own rules for the apron where the driveway meets the street, so check with the permit office before you pour that part.
4 inches for a garage that parks cars. If it will hold a heavy truck, a camper, or a workshop with heavy machines, pour 5 to 6 inches. The ground under it matters as much as the concrete, so compact the base well before the pour.
4 inches for a typical storage shed. A 10x10 shed floor at 4 inches takes about 1.23 cubic yards, or 56 80 lb bags. If you plan to park a riding mower or a small tractor inside, treat it like a garage floor.
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