How Thick Should a Concrete Slab Be?
Most slabs around a house are 4 inches thick. Driveways that carry heavy trucks, and pads under heavy equipment, go to 5 or 6 inches. Here is how to choose, what the building code says, and what each extra inch adds in concrete and cost.
Quick answer by project
| Project | Typical thickness | Notes |
|---|---|---|
| Patio, walkway, shed base, AC pad | 4 in | The usual choice for light, spread-out loads. |
| Driveway, passenger cars and SUVs | 4 in | Concrete Network and QUIKRETE both give 4 in as enough for cars. |
| Driveway with trucks, RVs or heavy vehicles | 5–6 in | Same sources; heavier loads need the extra depth. |
| Garage or house floor slab | at least 3.5 in by code | The residential code minimum; many builders pour 4 in or more. |
| Hot tub or heavy equipment pad | per the maker | Use the thickness and reinforcement the installation guide gives. |
What the building code says
The International Residential Code, which most US towns adopt in some form, sets a minimum of 3.5 inches for a concrete floor slab on the ground, and possibly more on expansive or difficult soils. It also calls for a 4 inch base of clean sand, gravel or crushed stone under slabs that sit below grade, and a vapor retarder under floors that will be heated. Driveways, walks and patios that won't be enclosed are exempt from the vapor retarder. Your local building department has the final word, so check before you pour anything structural.
Thicker edges where the load is
Slab edges carry wheel loads with less soil support around them. Concrete Network notes that contractors often thicken driveway edges by 1–2 inches, tapering back over about 4–8 inches from the edge. If you do that, add the thickened edge as its own part in the calculator so the extra concrete is counted.
What each extra inch costs
Concrete is ordered by volume, so thickness drives quantity directly. For a 12 × 20 ft slab with 10% extra:
| Thickness | Cubic yards | 80 lb bags | Ready-mix at $125/yd³ |
|---|---|---|---|
| 3.5 in | 2.85 | 129 | $356 |
| 4 in | 3.26 | 147 | $407 |
| 5 in | 4.07 | 184 | $509 |
| 6 in | 4.89 | 220 | $611 |
Going from 4 to 6 inches uses half as much concrete again. Installed prices rise too: This Old House puts the average at about $5 per square foot for a 4 inch slab and $6 for 6 inches.
The form-board trap
Lumber is sold by nominal size. A 2×4 is actually 3½ inches tall and a 2×6 is 5½ inches. A "4 inch" slab formed with 2×4s set on the ground comes out at 3½ inches unless the forms sit higher or the base is dug deeper. Decide the real thickness first, then enter that number when you work out the concrete.
Joints scale with thickness
Control joints let a slab crack in a straight line where you choose. The Portland Cement Association's rule of thumb is to space them at about twice the slab thickness, in feet, when the stone in the mix is under ¾ inch:
| Slab thickness | Joint spacing |
|---|---|
| 4 in | about 8 ft |
| 5 in | about 10 ft |
| 6 in | about 12 ft |
Checklist before you decide
- What will sit or drive on it, and how heavy is it?
- Does your local code or HOA set a minimum?
- Is the slab below grade or inside a heated space (base course and vapor retarder)?
- Will the edges carry wheels (thickened edge)?
- What is the real height of your form boards?
Sources
- International Residential Code slab requirements, summarized by EVstudio
- Concrete Network: how thick should a concrete driveway be
- QUIKRETE: concrete driveways and curbs
- Portland Cement Association: joints in concrete
- This Old House: concrete slab cost (March 2026)
- QUIKRETE Concrete Mix 1101 data sheet (bag yields)
Estimates for planning. Follow your plans, your supplier's instructions and your local building code.