How Much Concrete for a 20×20 Slab?
Quick Answer
For a 20×20 foot slab at 4-inch thickness, you need 4.94 cubic yards of concrete. At this volume, ready-mix delivery is strongly recommended over bags.
- 4 inches thick: 4.94 cubic yards (224 bags of 80 lb — use ready-mix instead)
- 5 inches thick: 6.17 cubic yards
- 6 inches thick: 7.41 cubic yards
- Ready-mix delivery: $600–$800 for 4-inch, $900–$1,200 for 6-inch
Formula
Multiply 20 ft × 20 ft × thickness in inches, divide by 12 to convert to feet, then divide by 27 for cubic yards.
Step-by-Step Calculation
- 1
Convert thickness to feet
4 inches ÷ 12 = 0.333 feet
- 2
Calculate cubic feet
20 × 20 × 0.333 = 133.3 cubic feet
- 3
Convert to cubic yards
133.3 ÷ 27 = 4.94 cubic yards
- 4
Add 10% waste
4.94 × 1.10 = 5.43 → order 5.5 cubic yards
20×20 Slab — Concrete by Thickness
| Thickness | Cubic Yards | Ready-Mix Cost* | With 10% Waste | Total Est. |
|---|---|---|---|---|
| 4 inches | 4.94 | $590–$790 | 5.5 CY | $660–$880 |
| 5 inches | 6.17 | $740–$990 | 6.8 CY | $815–$1,090 |
| 6 inches | 7.41 | $890–$1,185 | 8.2 CY | $985–$1,310 |
| 8 inches | 9.88 | $1,185–$1,580 | 10.9 CY | $1,310–$1,740 |
*Ready-mix concrete at $120–$160 per cubic yard delivered. Does not include labor or finishing.
Frequently Asked Questions
How many trucks of concrete for a 20×20 slab?
A standard concrete truck holds 8–10 cubic yards. A 4-inch thick 20×20 slab (4.94 CY + waste ≈ 5.5 CY) fits in one truck. A 6-inch thick slab may need a partial second load.
How much does a 20×20 concrete pad cost installed?
Professionally installed, a 20×20 slab (400 sq ft) costs $2,400–$4,800 for 4-inch thick or $3,600–$7,200 for 6-inch thick, including excavation, forming, concrete, and finishing.
Do I need a permit for a 20×20 concrete slab?
Most jurisdictions require permits for slabs over 200 sq ft. A 20×20 slab (400 sq ft) will almost certainly need a permit. Check with your local building department before starting work.