How many bags of concrete for a 20x20 slab?
A 4 in thick 20x20 slab needs 234 bags of 80 lb concrete.
That is 133.3 cubic feet (4.94 cubic yards), including 5% waste. In other bag sizes: 312 × 60 lb, or 467 × 40 lb.
At this size, order ready-mix instead. 4.94 cubic yards is 234 bags by hand — 18,720 lb of material to open, mix and place before the first of it sets. Why one cubic yard is the crossover.
A 20x20 slab at 4 in by the numbers
| Slab area | 400 sq ft |
|---|---|
| Concrete volume | 133.3 cu ft |
| In cubic yards | 4.94 cu yd |
| 80 lb bags | 234 |
| 60 lb bags | 312 |
| 40 lb bags | 467 |
| Dry weight, 80 lb bags | 18,720 lb |
| Compacted gravel base, 4 in | 4.94 cu yd |
| Cheaper as ready-mix? | yes |
Where that number comes from
- 20 ft × 20 ft = 400 sq ft
- 4 in thick = 0.3333 ft, so 400 × 0.3333 = 133.3 cubic feet
- An 80 lb bag yields 0.6 cu ft, so 133.3 ÷ 0.6 = 223 bags
- Add 5% for spillage and uneven subgrade = 234 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 400 sq ft that is roughly 10.0 cu ft — about 17 more 80 lb bags than the bare calculation gives you, on top of the 234 above.
Before you order: getting a slab pour right.
Other slab sizes
Concrete calculator for any size, or all calculators.