How Thick Should a Concrete Slab Be for a Patio, Garage or Shed?
Most residential patios and sidewalks are poured 4 inches thick, while garage floors and heavy driveways use 5 to 6 inches. A 12 by 12 foot patio needs 1.78 cubic yards at 4 inches, 2.22 at 5 and 2.67 at 6, so each extra inch adds about 25 percent over 4.
Typical thickness by use
Thickness is the biggest lever on how much concrete you buy, so it pays to pick it deliberately. Published guidance from The Concrete Network is consistent: most residential patio and sidewalk slabs are 4 inches thick, while driveway slabs may be 5 to 6 inches, and garage floors and heavy-duty driveways generally fall in the 5 to 6 inch range. The same source notes that most codes governing residential concrete driveways set a minimum of 4 inches over a well-graded, compacted gravel subbase.
| Slab | Typical thickness | Why |
|---|---|---|
| Patio, walkway | 4 in | Foot traffic and furniture only |
| Garage floor | 5 to 6 in | Cars, light trucks and point loads from jacks and stands |
| Shed floor | Check your plan and local code; 4 in is the common residential minimum | Depends on what the shed holds |
Treat the table as a starting point. Soil, frost depth, loads and local building rules vary, and a garage or any slab that carries a wall should follow your building department's requirements.
How much concrete each thickness needs
Volume scales directly with thickness. Going from 4 inches to 5 inches adds 25 percent more concrete, and going to 6 inches adds 50 percent. The formula is the same for every slab:
Here are exact volumes, with no waste allowance, for a patio, a two-car garage and a small shed pad:
| Slab | 4 in | 5 in | 6 in |
|---|---|---|---|
| 12 × 12 ft patio | 1.78 yd³ | 2.22 yd³ | 2.67 yd³ |
| 20 × 20 ft garage | 4.94 yd³ | 6.17 yd³ | 7.41 yd³ |
| 8 × 10 ft shed pad | 0.99 yd³ | - | 1.48 yd³ |
Run your own dimensions through the concrete calculator, which also converts to 40, 60 and 80-lb bags. As a rule of thumb, one cubic yard covers about 81 sq ft at 4 inches, according to Concrete Network's coverage chart, and about 54 sq ft at 6 inches.
Worked example: should the garage be 4 or 6 inches?
For the 20 × 20 ft garage, the step from 4 to 6 inches takes the order from 4.94 to 7.41 cubic yards, an extra 2.47 yards. That is the entire cost of the added thickness in material terms. Multiply 2.47 by your local delivered price per yard (get quotes, since prices vary widely by region) and compare it with what a cracked or settled floor would cost to fix. For a floor that will carry vehicles, most people find the extra yardage a small price for the margin.
The gravel base counts too
A slab sits on a compacted gravel subbase, and the base is a separate order. The depth depends on soil and local requirements, so confirm it for your site. As an illustration, if your plan calls for 4 inches of crushed stone under the 20 × 20 ft garage, that is about 6.91 tons at 1.4 tons per cubic yard before compaction. The gravel calculator handles other depths and stone types, and the cubic yard calculator converts any volume you already have.
Reinforcement and thickened edges
The Concrete Network article recommends wire mesh for 4 to 5 inch slabs and rebar for slabs 5 inches or thicker. Reinforcement helps control crack width but does not make a thin slab stronger. Many slabs are also poured with thickened edges, where the perimeter is made deeper than the interior, because edges carry the most stress. Add those extra inches to your volume. Ask your supplier or contractor what suits your soil and loads, and confirm anything structural with the local building department.
Common mistakes
- Using 4 inches for everything. It suits foot traffic. Vehicles and heavy equipment call for more.
- Forgetting that thickness varies. A "4-inch" slab poured on an uneven base ends up thicker in places, so add a 10 percent allowance, or up to 15 percent for rough ground, as Concrete Network suggests.
- Skipping the base. Pouring on loose or soft soil causes settling and cracks no matter how thick the slab is.
- Mixing units. Thickness is in inches, length and width in feet. Convert inches to feet before multiplying.
- Ignoring weight. A shed that stores a riding mower, a workshop or a vehicle is a garage as far as loads go. Size the slab to the heaviest thing it will hold.
What else to order or consider
Besides concrete and gravel, plan for forms, mesh or rebar, expansion or control joints, a vapor barrier if the space will be enclosed, and curing materials. Bagged mix works for small pads under about a yard, while larger pours are usually delivered by truck. Check with your local building department before you dig, since permits and inspections often apply to garage slabs and sheds above a certain size.
Calculators for this guide
Frequently asked questions
How thick should a concrete patio be?
Most residential patio and sidewalk slabs are 4 inches thick. A 12 by 12 foot patio at that thickness needs 1.78 cubic yards before waste.
How thick should a garage floor slab be?
Garage floors and heavy-duty driveways generally use 5 to 6 inches. A 20 by 20 foot garage needs 6.17 cubic yards at 5 inches or 7.41 at 6 inches, before waste. Confirm local code requirements first.
How much more concrete does a thicker slab take?
Volume scales with thickness, so 5 inches takes 25 percent more concrete than 4 inches, and 6 inches takes 50 percent more.
Sources and further reading
- How Thick Should a Concrete Driveway Be? — The Concrete Network
- How Many Square Feet in a Yard of Concrete? — The Concrete Network
- Concrete Calculator: how much do I need? — The Concrete Network
More guides
Estimates only. Always confirm quantities with your supplier or a qualified contractor before ordering, and add extra for waste and site conditions.