How Much Does a 20x20 Concrete Slab Cost?
A 20x20 concrete slab costs about 2,400 to 4,800 dollars installed in 2026, which works out to roughly 6 to 12 dollars per square foot for a standard 4-inch slab, with a national average near 8 dollars. That 400 square foot pour needs about 4.94 cubic yards of concrete, order 5.5 with waste. Below is the full breakdown by thickness, what drives the price up or down, the material versus labor split, and a calculator that sizes the job for you.
Key takeaways
- A 20x20 slab (400 sq ft) at 4 inches costs about 2,400 to 4,800 dollars installed in 2026, near 6 to 12 dollars per square foot.
- The pour needs about 4.94 cubic yards of concrete, but order 5.5 yards to cover waste and uneven ground.
- Stepping up to a 5-inch garage slab runs about 2,800 to 5,600 dollars; a 6-inch slab for heavy loads runs 3,200 to 6,400.
- Labor is 40 to 50% of the bill at 3 to 5 dollars per sq ft; ready-mix concrete itself is only about 690 to 910 dollars.
- Rebar adds 0.50 to 1.00 dollar per sq ft, a gravel base 0.80 to 1.50, and stamped finishes push past 8 dollars per sq ft.
The average cost of a 20x20 concrete slab in 2026
A 20 by 20 foot slab covers 400 square feet, and at the standard 4-inch thickness it costs about 2,400 to 4,800 dollars installed in most US markets in 2026. The national average sits around 8 dollars per square foot, so a typical mid-range pour lands near 3,200 dollars.
That price assumes competent site prep, light reinforcement like wire mesh, and a plain broom finish. Decorative work, thicker slabs, or difficult access push the total higher, while a DIY pour drops material cost under 1,600 dollars if you supply the labor yourself.
| Use case | Thickness | Installed cost (2026) |
|---|---|---|
| Patio, shed, light use | 4 in | 2,400 to 4,800 dollars |
| Residential garage floor | 5 in | 2,800 to 5,600 dollars |
| Heavy trucks, workshop | 6 in | 3,200 to 6,400 dollars |
| DIY, materials only | 4 in | about 1,200 to 1,600 dollars |
To price your own slab with your exact thickness and local rate, enter the dimensions in the concrete slab calculator and it returns yards, bags and an estimated cost.
How much concrete a 20x20 slab needs
A 20x20 slab at 4 inches thick needs about 4.94 cubic yards of concrete, which most contractors round up and order as 5.5 yards to cover waste and an uneven subgrade. The math is simple: 400 square feet times 0.333 feet of thickness is 133 cubic feet, divided by 27 gives 4.94 cubic yards.
Cubic yards = Length(ft) x Width(ft) x (Thickness(in) ÷ 12) ÷ 27Thickness changes the volume a lot. A 5-inch slab needs about 6.17 yards, and a 6-inch slab needs about 7.41 yards, so going from 4 to 6 inches uses 50% more concrete and raises both material and labor cost.
| Thickness | Cubic yards needed | Order with waste | 80 lb bags (DIY) |
|---|---|---|---|
| 4 in | 4.94 | 5.5 | about 222 |
| 5 in | 6.17 | 7 | about 278 |
| 6 in | 7.41 | 8 | about 333 |
Labor versus materials: where the money goes
On a slab this size, the bill splits fairly evenly, with labor running 40 to 50% of the total and materials the rest. Concrete installers charge about 3 to 5 dollars per square foot for forming, placing, screeding and finishing, so the labor alone on 400 square feet is roughly 1,200 to 2,000 dollars.
The ready-mix concrete itself is a smaller share than people expect. At 125 to 165 dollars per cubic yard delivered in 2026, the 5.5 yards for a 4-inch slab cost about 690 to 910 dollars in material.
The gap between material and total is why DIY tempts people, but a 5.5-yard pour has a 60 to 90 minute working window and needs a crew of three or four to place and finish before it sets. To weigh hiring out against pouring yourself, the breakdown in cost to pour a concrete slab walks through the labor side in detail.
What drives the price up: rebar, base and site prep
The same 20x20 slab can swing 2,000 dollars depending on what goes under and inside it. Reinforcement is the first variable: wire mesh adds about 0.40 to 0.60 dollars per square foot, while a rebar grid runs 0.50 to 1.00 dollars per square foot and is required by code for garage floors and any slab carrying vehicles.
A compacted gravel base adds 0.80 to 1.50 dollars per square foot and is not optional in cold climates, since slabs heave and crack within a winter or two without 4 to 6 inches of compacted base. Skipping it is the most expensive way to save money on a pour.
Site prep is the wild card. Excavation, grading and drainage run 10 to 25% of the total and depend on slope and soil, with significant slope or drainage fixes adding 500 to 3,000 dollars. A permit for foundation work adds 50 to 200 dollars, and some areas require soil testing first.
Finishes, region and other add-ons
A plain broom finish is the baseline price. Decorative options add up fast: stamped concrete costs about 8 to 19 dollars per square foot installed, and on a 400 square foot slab that can more than double the total compared with a broom finish.
Location matters as much as spec. High-cost coastal markets like California, New York and the Pacific Northwest push standard work toward 12 to 18 dollars per square foot, while the Southeast, Midwest and inland Southwest stay in the 6 to 12 dollar range. The same 20x20 slab can cost 2,800 dollars in Tennessee and 4,200 to 5,600 in California for identical specs.
Other add-ons to budget for include a pump truck at 150 to 300 dollars per hour when the mixer cannot reach the pour, and demolition of an old slab at 2 to 6 dollars per square foot. For a thicker garage or driveway pour, the figures in how much a concrete driveway costs cover the heavier specs that apply.
How to get an accurate price for your slab
The most reliable way to budget is to nail down the spec first, then get three written bids to the same scope. A bid significantly below 5.50 dollars per square foot in an average market deserves scrutiny, because someone is cutting the base, the reinforcement, or the thickness.
Before you call contractors, know your own numbers so you can read the quotes. Multiply your length, width and thickness, divide by 27 for cubic yards, add 10% for waste, then apply a per-yard concrete price and a per-square-foot labor rate.
Rather than juggle the conversions by hand, the concrete slab calculator takes your dimensions and returns cubic yards, bag counts, and an estimated installed cost, so you walk into the conversation knowing what the job should run.
Frequently asked questions
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References
- HomeGuide: Concrete Slab Cost (2026)
- NRMCA: Ready Mixed Concrete
- ACI 332: Residential Concrete Code
- Concrete (Wikipedia)