Quick Answer: 20×20 Slab
- At 4 inches: 4.94 cubic yards → order 5.5 yd³ (with 10% waste)
- At 5 inches: 6.17 cubic yards → order 6.8 yd³
- At 6 inches: 7.41 cubic yards → order 8.2 yd³
The Formula
20×20 slab at 4 inches:
20 × 20 × 4 ÷ 324 = 4.94 cubic yards
Add 10% waste: 4.94 × 1.10 = 5.43 yd³ (round up to 5.5)
Concrete by Common Slab Size (4" and 6" Thick)
| Slab Size | Sq Ft | 4" Thick (yd³) | 6" Thick (yd³) | 80 lb Bags (4") |
|---|---|---|---|---|
| 10×10 ft | 100 | 1.23 | 1.85 | 67 bags |
| 12×12 ft | 144 | 1.78 | 2.67 | 97 bags |
| 16×16 ft | 256 | 3.16 | 4.74 | 172 bags |
| 20×20 ft | 400 | 4.94 | 7.41 | 269 bags |
| 24×24 ft | 576 | 7.11 | 10.67 | 387 bags |
| 24×30 ft | 720 | 8.89 | 13.33 | 484 bags |
| 30×30 ft | 900 | 11.11 | 16.67 | 605 bags |
| 40×40 ft | 1,600 | 19.75 | 29.63 | 1,076 bags |
* All values include 10% waste factor. 80 lb bag = 0.60 cu ft = 0.022 yd³
Which Thickness Do I Need?
| Use | Thickness | Why |
|---|---|---|
| Walkway / sidewalk | 4" | Foot traffic only |
| Patio (residential) | 4" | Foot traffic, outdoor furniture |
| Garage floor (car) | 4–5" | Standard vehicle weight |
| Driveway (cars) | 4–6" | Vehicle entry and parking |
| Heavy trucks / RV pad | 6–8" | Higher load-bearing need |
A 20×20 slab at 4 inches = 4.94 yd³. That's 269 bags of 80 lb mix to hand-mix — physically exhausting and impractical. Ready-mix delivered in a truck is the right choice for any slab larger than 10×10 ft.
Calculate Concrete for Any Slab Size
Enter exact dimensions and thickness for cubic yards, bags, and cost estimate.
Open Slab Calculator →FAQ
How many cubic yards of concrete for a 20×20 slab?
At 4 inches thick: 20 × 20 × 4 ÷ 324 = 4.94 yd³. With 10% waste: 5.5 yd³. At 6 inches: 7.41 yd³ → order 8.2 yd³.
How many 80 lb bags of concrete for a 20×20 slab?
At 4 inches thick: 269 bags of 80 lb concrete. This is impractical to hand-mix — strongly consider ordering ready-mix concrete from a truck instead.
How much does a 20×20 concrete slab cost?
Materials only (ready-mix at $150/yd³): 5.5 yd³ × $150 = ~$825. Full installation with labor typically runs $3,000–$6,000 for a 20×20 slab (surface prep, forming, finishing, and sealing included).