Key takeaways
- The 2021 IRC sets a 3½-inch minimum for concrete slab-on-ground floors.
- That's for floors covered by the code, and other projects or soils can need more.
- Each extra inch adds about 0.31 cubic yards per 100 square feet.
For slab-on-ground floors, the 2021 International Residential Code (IRC) sets a minimum of 3½ inches, and points to separate rules for expansive soils. Your project may need more, so treat this as a starting point.
What depends on your project
Thickness depends on what the slab carries and what's under it. A floor, a shed base, a patio and a driveway can have different needs, as can reinforced slabs and poor or expansive soils.
Your local building department can tell you what it requires, including any permit. If the slab will hold vehicles, heavy equipment or a structure, ask them or an engineer before you pour.
QUIKRETE says its Concrete Mix is designed for pours 2 inches thick or more. That's a limit for the product, not a recommended slab thickness.
How thickness changes what you buy
For every 100 square feet of slab:
| Thickness | Cubic feet | Cubic yards | 80 lb bags |
|---|---|---|---|
| 3½ in | 29.2 | 1.08 | 49 |
| 4 in | 33.3 | 1.23 | 56 |
| 5 in | 41.7 | 1.54 | 70 |
| 6 in | 50.0 | 1.85 | 84 |
These have no extra allowance. Use the concrete slab calculator for your exact size.
Measure the real depth
Your concrete volume depends on the actual depth of the forms, and ground that dips can use more. Check depth in several places before ordering, and consider raising your allowance if the base is uneven.
Decide who to ask
| Your project | Start with |
|---|---|
| A floor inside a building | Your building department, starting from the IRC's 3 1/2 in floor minimum |
| A shed base or walkway | Your building department, for any permit or local rule |
| A driveway or garage | Your building department or a concrete contractor |
| Heavy equipment, or expansive or poor soil | An engineer |
Frequently asked questions
Is 3½ inches enough for a slab?
It's the 2021 IRC minimum for slab-on-ground floors covered by that section. It doesn't mean it's right for every use, such as a driveway or a slab holding heavy loads.
How thick should a driveway be?
We haven't found a single national rule we can cite. Requirements and good practice depend on your soil, vehicles and location, so ask your building department or a concrete contractor.
Does a thicker slab cost much more?
Going from 4 to 5 inches uses 25% more concrete. Labor and base preparation may also change, so ask for a quote at both thicknesses.
Sources
We use primary sources where we can: building codes, federal rules and manufacturer documents. Read our editorial standards.
- 2021 International Residential Code, Chapter 5, Section R506.1
- QUIKRETE: Concrete Mix data sheet, product 1101