How Much Does a Concrete Slab Cost?
Quick answer: a basic 4-inch slab runs about $6–$10 per square foot installed. A 20×20 slab (400 sq ft) is typically $2,400–$4,000. Here's what drives the price and how to estimate yours.
- Cost per square foot
- What's included
- How much concrete
- Worked example
- Rebar & finish upgrades
- Common mistakes
Cost per square foot
A 4-inch concrete slab typically costs $6–$10 per sq ft fully installed. The range depends on your region, site access, soil prep, and finish. Broom-finish exterior slabs sit at the low end; interior slabs with a smooth trowel finish run higher. Get a local quote rather than trusting a national number — labor varies a lot by area.
What's included
The installed price covers site clearing, forms, rebar or wire, pouring, and finishing. What it usually doesn't cover: grading and drainage work, demolition of an old slab, or permits. If the quote seems low, ask exactly what's included before you commit — a "$5/sq ft" price can turn into $12 by the time site prep is added.
How much concrete
| Slab size | Sq ft | 4 in thick (cu yd) | 6 in thick (cu yd) |
|---|---|---|---|
| 10×10 | 100 | 1.2 | 1.9 |
| 12×12 | 144 | 1.8 | 2.7 |
| 20×20 | 400 | 4.9 | 7.4 |
| 30×40 | 1,200 | 14.8 | 22.2 |
Cubic yards = sq ft × (thickness in ft) ÷ 27. Ready-mix is ordered in full cubic yards, so round up to the next yard.
Worked example
A 20×20 slab at 4 in thick:
- Area = 400 sq ft; volume = 400 × 0.333 ÷ 27 = 4.9 cu yd → order 5 cu yd
- Concrete material ≈ 5 × $140/yd ≈ $700
- Labor, forms, rebar, finish ≈ $1,800–$3,300
- Total ≈ $2,400–$4,000 installed
That matches the $6–$10 per sq ft range once everything is added.
Rebar & finish upgrades
Decide on reinforcement before pricing. For driveways and slabs under load, rebar (or at least wire mesh) is worth the cost; for a small garden pad, it may be optional. Smooth-trowel interior finish, colored concrete, and stamped patterns each add to the per-sq-ft price — get them itemized so you can compare quotes fairly.
Common mistakes
- Quoting material only. Labor is usually the bigger half of the bill.
- Forgetting site prep. Grading, gravel base, and access can exceed the concrete cost.
- Ordering short. Ready-mix rounds to full yards — order up or pay for a partial load.
- No written scope. "What's included" needs to be in writing.
FAQ
A basic 4-inch concrete slab runs about $6–$10 per square foot installed. A 20×20 slab (400 sq ft) typically costs $2,400–$4,000 depending on site prep, rebar, and finish.
A 20×20 slab at 4 inches thick needs about 5 cubic yards of concrete (400 sq ft × 4 in ÷ 324). At 6 inches thick, about 7.5 cubic yards.
For a 4-inch slab on stable soil, a wire mesh or minimal reinforcement is common. Rebar matters more for driveways, patios under load, or slabs over poor soil. Ask your contractor what the local code requires.
Labor typically makes up 40–60% of the total, with site prep, forms, and finishing the main cost drivers. Ready-mix concrete itself is a smaller share than most people expect.