How many bags of concrete for a 30x40 slab?
A 4 in thick 30x40 slab needs 701 bags of 80 lb concrete.
That is 400.0 cubic feet (14.81 cubic yards), including 5% waste. In other bag sizes: 934 × 60 lb, or 1401 × 40 lb.
At this size, order ready-mix instead. 14.81 cubic yards is 701 bags by hand — 56,080 lb of material to open, mix and place before the first of it sets. Why one cubic yard is the crossover.
A 30x40 slab at 4 in by the numbers
| Slab area | 1200 sq ft |
|---|---|
| Concrete volume | 400.0 cu ft |
| In cubic yards | 14.81 cu yd |
| 80 lb bags | 701 |
| 60 lb bags | 934 |
| 40 lb bags | 1401 |
| Dry weight, 80 lb bags | 56,080 lb |
| Compacted gravel base, 4 in | 14.81 cu yd |
| Cheaper as ready-mix? | yes |
Where that number comes from
- 30 ft × 40 ft = 1200 sq ft
- 4 in thick = 0.3333 ft, so 1200 × 0.3333 = 400.0 cubic feet
- An 80 lb bag yields 0.6 cu ft, so 400.0 ÷ 0.6 = 667 bags
- Add 5% for spillage and uneven subgrade = 701 bags
Yields are finished volume, not dry weight. What the 5% margin is really for.
Working to a different size? The calculator takes any dimensions and returns the same figures.
The one that catches people on this size
Forms settle and subgrade dips, so a slab "4 inches thick" usually consumes 5-10% more than the arithmetic says. On 1200 sq ft that is roughly 30.0 cu ft — about 50 more 80 lb bags than the bare calculation gives you, on top of the 701 above.
Before you order: getting a slab pour right.
Other slab sizes
Concrete calculator for any size, or all calculators.