How Much Concrete for a 20 × 30 Slab?
A 20 × 30 ft slab is 600 sq ft. It needs 8.15 cubic yards of concrete at 4 inches thick or 12.22 yd³ at 6 inches, including 10% extra.
| Order | 4 in thick | 6 in thick |
|---|---|---|
| Cubic yards | 8.15 | 12.22 |
| 80-lb bags | 367 | 550 |
| 60-lb bags | 489 | 734 |
| 50-lb bags | 587 | 880 |
| 40-lb bags | 734 | 1,100 |
| Cubic meters | 6.23 | 9.34 |
Concrete needed
8.15 cubic yards
220.0 ft³ · 6.23 m³ · includes 10% extra
You need about 8.15 cubic yards of concrete, or 367 bags of 80 lb mix.
Bags of premix
- 73440 lb bags
- 58750 lb bags
- 48960 lb bags
- 36780 lb bags
- —20 kg bags
- —25 kg bags
- Before extra7.41 yd³ (200.0 ft³)
- Weight of bags29,360 lb
- Per hole—
- Ready-mix costAdd a price per yd³
- Bagged costAdd a price per bag
One 80 lb bag makes about 0.60 ft³ (manufacturer's published yield). Bags are rounded up.
At 8.15 yd³, bags are not realistic (367 × 80 lb). This is a ready-mix pour, often with a contractor's crew and sometimes a pump.
Estimate only, not a quote or engineering advice. Disclaimer
A 20 × 30 ft slab fits a deeper two-car garage with a workbench, or a large shop floor.
20 × 30 slab at other thicknesses
| Thickness | yd³ exact | yd³ +10% | 80-lb bags | 60-lb bags |
|---|---|---|---|---|
| 3.5 in | 6.48 | 7.13 | 321 | 428 |
| 4 in | 7.41 | 8.15 | 367 | 489 |
| 5 in | 9.26 | 10.19 | 459 | 612 |
| 6 in | 11.11 | 12.22 | 550 | 734 |
| 8 in | 14.81 | 16.30 | 734 | 978 |
How it's worked out
For bags, the 220.0 ft³ (with extra) is divided by each bag's yield: 0.60 ft³ for 80 lb, 0.45 for 60 lb, 0.375 for 50 lb and 0.30 for 40 lb, then rounded up. At 4 inches the slab itself weighs about 28,000 lb, using the 140 lb/ft³ unit weight QUIKRETE lists for its Concrete Mix.
Bags or a truck?
At 8.15 yd³, bags are not realistic: it would take 367 × 80-lb bags. This is a ready-mix pour, and at this size often a job for a concrete contractor with a crew, and sometimes a pump.
For a slab this size, the number to give a ready-mix supplier is the yardage: 8.15 yd³ at 4 inches or 12.22 yd³ at 6 inches, with 10% extra already included. Some people order a little less than the full allowance and keep a few bags on hand to top off.
Don't forget the base: most slabs sit on a compacted layer of gravel, and thicker edges or footings around the perimeter add concrete. Use the footing calculator for a thickened edge.
Frequently asked questions
How many yards of concrete for a 20x30 slab?
7.41 cubic yards at 4 inches thick and 11.11 yd³ at 6 inches, before waste. With 10% extra, order 8.15 or 12.22 yd³.
How many bags of concrete for a 20x30 slab?
At 4 inches with 10% extra: 367 bags of 80-lb mix, 489 of 60 lb, 587 of 50 lb or 734 of 40 lb. At 6 inches: 550 bags of 80 lb or 734 of 60 lb.
How many cubic feet of concrete for a 20x30 slab?
200.0 ft³ at 4 inches (600 sq ft × 4 ÷ 12) and 300.0 ft³ at 6 inches, before waste.
How thick should a 20x30 slab be?
Four inches is usual for patios, walkways and shed pads. Garage floors, driveways and pads for heavy loads are commonly 5 to 6 inches. Follow local code or your plans for anything structural.
Other slab sizes
Related
Sources
- QUIKRETE: Concrete Mix No. 1101 data sheet (Revised Oct 2022; checked Nov 11, 2026)