Concrete Floor Thickness
The minimum concrete floor thickness in a house is 3-1/2 inches under the International Residential Code, and most house and basement floors are poured at 3-1/2 to 4 inches. A garage floor for cars and pickups should be 4 inches on firm, well-prepared ground and 4-1/2 to 6 inches on very soft ground, based on ACI's parking lot guide. Go thicker for concentrated loads such as a car lift, not for the weight of the car.
Thickness by use
House floor slab on ground
3-1/2 to 4 in
3-1/2 in is the code minimum
- Basis
- IRC R506.1
Basement floor below grade
3-1/2 to 4 in
on a 4 in base course
- Basis
- IRC R506
Garage floor cars and pickups
4 to 6 in
4 in on good ground
- Basis
- ACI 330R-08
Car lift two-post
12 in
new slab, per the lift maker
- Basis
- BendPak
Pouring outdoors instead? A patio, driveway or exterior pad is covered in how thick a concrete slab should be, which also explains how to measure the thickness of a slab that's already poured.
What the code says
Section R506.1 of the International Residential Code, shown here in the 2020 Indiana Residential Code, requires concrete slab-on-ground floors to be at least 3-1/2 inches thick. That's also the actual depth of a 2x4, which the American Softwood Lumber Standard (PS 20) sets at 1-1/2 by 3-1/2 inches dry. Screed a slab to the top of 2x4 forms and you get the code minimum, which is why a "4 inch" floor is often 3-1/2.
ACI's parking lot guide, ACI 330R-08 (since superseded by ACI PRC-330-21), sets a practical floor of its own: "As a practical matter, there is no benefit in building slabs less than 4 in. (100 mm) thick." ACI's floor construction guide, ACI 302.1R-04, doesn't give a default thickness. It lists concrete thickness among the items the designer should specify.
Garage floor thickness
ACI 330R-08 Table 3.4 gives 20-year design thicknesses for car parking areas (traffic category A). Two things set the answer: how firm the ground is, measured as the modulus of subgrade reaction k, and the concrete's flexural strength (MOR).
| Subgrade k (psi/in) | 1 truck a day | 10 trucks a day |
|---|---|---|
| 500 | 4 | 4 to 4.5 |
| 300 | 4 to 4.5 | 4 to 4.5 |
| 100 | 4 to 5 | 4.5 to 5.5 |
| 50 | 4.5 to 5.5 | 5 to 6 |
Each range runs from 650 psi flexural strength (thinner) to 500 psi (thicker). A home garage sits at the light end, so on firm, well-compacted ground 4 inches is the answer. Good subgrade preparation is what keeps you from needing 6. ACI 330R also says "a compressive strength of at least 4000 psi (28 MPa) should be specified" for heavy traffic loads or when durability is critical, which matters in climates where road salt drips off the car. See the concrete PSI guide.
Lifts, jack stands and point loads
A tire spreads the car's weight over a patch of slab. A jack stand, a lift post or a loaded pallet jack wheel puts it on a few square inches, and that's what cracks garage floors. A car lift comes with its own concrete requirement. BendPak's slab requirements call for a new slab poured for a lift to be at least 12 inches thick, 4,000 psi and cured 28 days. For an existing floor, its minimums are 4-1/4 inches of 3,000 psi concrete for its lighter two-post lifts, 6-1/2 inches for the XPR-12 and 8 inches for the XPR-15 and XPR-18, with anchors at least 6 inches from any joint. Follow your lift's manual, not a thickness chart. Finishes and coatings are on the concrete garage floors page.
What goes under the slab
Base course
The IRC calls for a 4 inch base of clean graded sand, gravel or crushed stone where the slab is below grade, as in a basement. Fill under the slab must be free of vegetation and compacted for uniform support. Work out the tonnage with the gravel calculator.
Vapor retarder
IRC R506.2.3 requires 6 mil polyethylene or an approved retarder, joints lapped at least 6 inches, between the slab and the base. Garages and unheated accessory buildings are exempt. Retarder classes and insulation are on the under-slab insulation page.
Reinforcement
The IRC doesn't require it, but where it's used it must stay "from the center to upper one-third of the slab" during the pour. ACI 302.1R-04 puts welded wire 2 inches below the surface or in the upper third, whichever is closer to the top. See the rebar calculator and rebar and reinforcement.
Even support beats a thick, costly base. ACI 330R-08 says "Uniform subgrade support is the goal of site preparation," and that it "probably is not economical to use imported subbase material or to chemically treat the subgrade for the sole purpose of increasing k values."
What an extra inch costs
A cubic yard covers 324 ÷ thickness in inches square feet. For a 24 by 24 ft garage floor:
| Thickness | Sq ft per yd | Cubic yards | More than 3-1/2 in |
|---|---|---|---|
| 3-1/2 in | 93 | 6.22 | None |
| 4 in | 81 | 7.11 | +0.89 yd (14%) |
| 5 in | 65 | 8.89 | +2.67 yd (43%) |
| 6 in | 54 | 10.67 | +4.44 yd (71%) |
At our default of $160 a yard delivered (change it in the cost calculator to your local quote), the concrete runs about $1.98 a square foot at 4 inches, $2.47 at 5 inches and $2.96 at 6 inches. The step from 3-1/2 to 4 inches is less than a yard on this floor; the step to 6 is more than four. Run your own size through the concrete floor calculator, and see concrete floor cost for labor and finishes.
Pouring it
Prepare and compact the base, lay the vapor retarder, set forms to the finished height, support any steel, then place, screed, float and finish, and cut the joints. How to pour a concrete slab covers each step, and concrete cure time covers when you can walk, park or lay flooring on it. Fixing an existing floor instead? See floor leveling or resurfacing, or the full concrete floors guide.
Frequently asked questions
How thick should a concrete floor be?
At least 3-1/2 inches in a house, which is the minimum in IRC Section R506.1, and most floors are poured at 3-1/2 to 4 inches. A 2x4 form is 3-1/2 inches deep, which is why a "4 inch" floor is often 3-1/2. Go thicker only for a reason, usually a heavy point load.
How thick should a garage floor be?
Four inches on firm, well-prepared ground. ACI 330R-08 (since superseded by ACI PRC-330-21) Table 3.4 gives 4 inches for car parking at a subgrade k of 500, rising to 4-1/2 to 6 inches at a k of 50, depending on concrete strength and truck traffic. A car lift sets its own minimum: BendPak calls for a new slab at least 12 inches thick, and on an existing floor 4-1/4 inches for its lighter two-post lifts and 8 inches for its heaviest.
How thick are basement floors?
Usually 3-1/2 to 4 inches, the same code minimum as other house floors. Because a basement slab is below grade, the IRC also calls for a 4 inch base of clean graded sand, gravel or crushed stone under it, plus a 6 mil vapor retarder.
Does a concrete floor need rebar?
Not by the IRC. Where reinforcement is used, the code says it must stay from the center to the upper one-third of the slab during the pour, and ACI 302.1R-04 puts welded wire 2 inches below the surface or in the upper third, whichever is closer to the top. Mesh left lying on the base does little.
What does an extra inch of concrete cost?
On a 24 by 24 ft floor, going from 3-1/2 to 4 inches adds 0.89 cubic yards. Going to 6 inches adds 4.44 yards, 71 percent more concrete. The concrete cost rises in the same proportion, before labor. A cubic yard covers 324 divided by the thickness in inches, in square feet.