How Much Concrete for a 30 × 40 Slab?
A 30 × 40 ft slab is 1,200 sq ft. It needs 16.30 cubic yards of concrete at 4 inches thick or 24.44 yd³ at 6 inches, including 10% extra.
| Order | 4 in thick | 6 in thick |
|---|---|---|
| Cubic yards | 16.30 | 24.44 |
| 80-lb bags | 734 | 1,100 |
| 60-lb bags | 978 | 1,467 |
| 50-lb bags | 1,174 | 1,760 |
| 40-lb bags | 1,467 | 2,200 |
| Cubic meters | 12.46 | 18.69 |
Concrete needed
16.30 cubic yards
440.0 ft³ · 12.46 m³ · includes 10% extra
You need about 16.30 cubic yards of concrete, or 734 bags of 80 lb mix.
Bags of premix
- 1,46740 lb bags
- 1,17450 lb bags
- 97860 lb bags
- 73480 lb bags
- —20 kg bags
- —25 kg bags
- Before extra14.81 yd³ (400.0 ft³)
- Weight of bags58,720 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 16.30 yd³, bags are not realistic (734 × 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 30 × 40 ft slab is a popular shop or pole-barn floor. Plan the pour with your supplier: it is well beyond a single small load.
30 × 40 slab at other thicknesses
| Thickness | yd³ exact | yd³ +10% | 80-lb bags | 60-lb bags |
|---|---|---|---|---|
| 3.5 in | 12.96 | 14.26 | 642 | 856 |
| 4 in | 14.81 | 16.30 | 734 | 978 |
| 5 in | 18.52 | 20.37 | 917 | 1,223 |
| 6 in | 22.22 | 24.44 | 1,100 | 1,467 |
| 8 in | 29.63 | 32.59 | 1,467 | 1,956 |
How it's worked out
For bags, the 440.0 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 56,000 lb, using the 140 lb/ft³ unit weight QUIKRETE lists for its Concrete Mix.
Bags or a truck?
At 16.30 yd³, bags are not realistic: it would take 734 × 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: 16.30 yd³ at 4 inches or 24.44 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 30x40 slab?
14.81 cubic yards at 4 inches thick and 22.22 yd³ at 6 inches, before waste. With 10% extra, order 16.30 or 24.44 yd³.
How many bags of concrete for a 30x40 slab?
At 4 inches with 10% extra: 734 bags of 80-lb mix, 978 of 60 lb, 1,174 of 50 lb or 1,467 of 40 lb. At 6 inches: 1,100 bags of 80 lb or 1,467 of 60 lb.
How many cubic feet of concrete for a 30x40 slab?
400.0 ft³ at 4 inches (1,200 sq ft × 4 ÷ 12) and 600.0 ft³ at 6 inches, before waste.
How thick should a 30x40 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)