How thick a concrete slab should be depends on what sits on it, what is under it and what your local code requires. This guide gives the usual thicknesses for patios, sheds, garages, driveways and house floors, the code minimums that apply in most of the US, and how each extra inch changes the amount of concrete you have to buy. When you know the size and thickness, use the concrete slab calculator for the exact volume and number of bags.
Slab thickness by use
| Use | Typical thickness | Notes |
|---|---|---|
| Walkway, patio, shed floor (light use) | 4 in | The common minimum for flatwork on a prepared base. A 4 in slab is the default in our concrete calculator. |
| House floor (slab-on-ground) | 3-1/2 in minimum by code | IRC R506.1 sets 3-1/2 in as the minimum for concrete floors on ground; many builders pour 4 in. Over a 4 in base and a vapour retarder (R506.2). |
| Garage floor, cars and light trucks | 4 in, often 5 in | Thicker slabs and higher-strength, air-entrained concrete are common where winters are severe or loads are heavy. |
| Driveway | 4 in for cars, 5 to 6 in for heavier vehicles | Edges and the apron where trucks turn take the worst loads; many builders thicken them. |
| Hot tub pad | As the spa maker specifies | A filled spa can weigh several tons. Makers state the pad thickness and strength they require; follow the installation manual. |
| Car lift, heavy machinery | As the equipment maker specifies | Anchors need a minimum thickness and concrete strength stated by the maker; a typical garage floor may not qualify. Check before buying. |
What each inch costs: concrete per 100 sq ft
| Thickness | Volume | Cubic yards | 80 lb bags | 60 lb bags | Weight of the slab |
|---|---|---|---|---|---|
| 3-1/2 in | 29.2 cu ft | 1.08 cu yd | 49 | 65 | 4,229 lb |
| 4 in | 33.3 cu ft | 1.23 cu yd | 56 | 75 | 4,833 lb |
| 5 in | 41.7 cu ft | 1.54 cu yd | 70 | 93 | 6,042 lb |
| 6 in | 50.0 cu ft | 1.85 cu yd | 84 | 112 | 7,250 lb |
| 8 in | 66.7 cu ft | 2.47 cu yd | 112 | 149 | 9,667 lb |
Going from 4 to 5 inches adds 25% more concrete, and from 4 to 6 adds 50%. On a 24 x 24 ft garage that is the difference between about 7.1 and 8.9 cubic yards, almost 1.8 cubic yards more. That is why thickness is worth deciding on purpose rather than by habit: the extra inch is cheap insurance under a heavy load, and wasted money under a garden shed.
What is under the slab matters as much as thickness
- Subgrade. Remove topsoil, roots and soft spots, and compact the soil. A thick slab on soft or uneven ground cracks sooner than a thinner slab on firm ground.
- Base. For house floors the IRC calls for a 4 in base of clean graded sand, gravel or crushed stone under the slab unless the soil is a well-drained type (R506.2.2). The same base is good practice for garages and patios.
- Vapour retarder. Under a heated floor the IRC requires a 6-mil polyethylene or approved vapour retarder with lapped joints (R506.2.3), so ground moisture does not come up through the slab into floor coverings.
- Reinforcement. Wire mesh or rebar controls crack width; it does not stop cracking. Bar sizes and weights are in our rebar size chart, and the rebar calculator lays out a grid.
Control joints: spacing follows thickness
Concrete shrinks as it cures, and a slab without joints will crack where it chooses. The common guidance from the concrete industry is to cut or tool control joints at a spacing of roughly 24 to 36 times the slab thickness, to a depth of about one quarter of the thickness. For a 4 in slab that is joints every 8 to 12 ft, about 1 in deep; for a 6 in slab, every 12 to 18 ft and 1-1/2 in deep. Keep panels close to square (the long side no more than about 1.5 times the short side), and cut saw joints as soon as the concrete is hard enough not to ravel, usually within the first day.
Worked example: a 12 x 16 ft shed slab
At 4 in: 12 × 16 × 4 ÷ 12 = 64 cu ft, or 2.37 cu yd. In 80 lb bags that is 107 bags before waste; add 5 to 10% for an uneven base, so about 116 bags, which weigh around 4.6 tons. At that size, ready-mix delivery is usually easier than mixing bags by hand. A 4 in slab of this size would normally have one control joint across the middle, making two 12 x 8 ft panels.
Safety and code
Footings, foundation walls and structural slabs are designed to the building code and, where required, by an engineer; this page is about slabs on ground. Minimum concrete strength depends on weathering exposure and use (IRC R402.2), and garage floors in cold climates need air-entrained concrete to survive freeze-thaw cycles and road salt. Ask your building department which requirements apply, and whether a permit is needed for your slab.
Frequently asked questions
How thick should a concrete patio be?
4 in is the usual thickness for a patio or walkway on a compacted gravel base. Go thicker only for heavy loads such as a hot tub or vehicles.
How thick should a garage floor slab be?
At least 4 in, and many builders pour 5 in or thicken the edges and apron. In severe-weather areas use air-entrained concrete of the strength your local code requires.
What is the minimum thickness of a house slab?
The IRC sets 3-1/2 in as the minimum for concrete floors on ground (R506.1), over a 4 in base and a vapour retarder in most cases (R506.2).
How much concrete is in a 4 inch slab?
About 1.23 cubic yards per 100 sq ft, or 34 bags of 80 lb premix. Use the concrete slab calculator for your exact size.
Does a thicker slab need rebar?
Thickness and reinforcement do different jobs: thickness carries load, reinforcement holds cracks tight. Many slabs use both; follow your plans or local practice.
Sources
- International Code Council, 2021 International Residential Code (IRC). Requirements are paraphrased with their section numbers; local codes and amendments may differ, so confirm with your building department. Sections R506.1 (concrete floor thickness), R506.2.2 (base), R506.2.3 (vapour retarder), R402.2 (concrete strength).
- QUIKRETE Concrete Mix product data: yields of about 0.60 cu ft per 80 lb bag and 0.45 cu ft per 60 lb bag (the same figures used by our concrete calculators).
- Control joint spacing and depth: common concrete industry guidance (American Concrete Institute and Portland Cement Association publications on slabs on ground), summarised.
Last reviewed: 24 October 2026. Numbers in the tables are our arithmetic from the sources listed; code sections are summarised, not quoted. How we check our work: methodology and corrections. Spotted an error? Please let us know.