How many bags of concrete by slab size
A 10x10 slab at 4 in takes 59 bags of 80 lb.
That is 33.3 cubic feet, 1.23 cubic yards — and already past the point where ready-mix is usually the better buy.
Every slab size, at 4 in thick
| Slab | Sq ft | Cu ft | Cu yd | 80 lb | 60 lb | 40 lb | Buy |
|---|---|---|---|---|---|---|---|
| 8x8 | 64 | 21.3 | 0.79 | 38 | 50 | 75 | bags |
| 8x10 | 80 | 26.7 | 0.99 | 47 | 63 | 94 | bags |
| 10x10 | 100 | 33.3 | 1.23 | 59 | 78 | 117 | ready-mix |
| 10x12 | 120 | 40.0 | 1.48 | 71 | 94 | 141 | ready-mix |
| 12x12 | 144 | 48.0 | 1.78 | 84 | 112 | 168 | ready-mix |
| 12x16 | 192 | 64.0 | 2.37 | 113 | 150 | 225 | ready-mix |
| 12x20 | 240 | 80.0 | 2.96 | 141 | 187 | 281 | ready-mix |
| 16x16 | 256 | 85.3 | 3.16 | 150 | 200 | 299 | ready-mix |
| 20x20 | 400 | 133.3 | 4.94 | 234 | 312 | 467 | ready-mix |
| 24x24 | 576 | 192.0 | 7.11 | 336 | 448 | 672 | ready-mix |
| 30x40 | 1200 | 400.0 | 14.81 | 701 | 934 | 1401 | ready-mix |
Counts include 5% for spillage and the low spots every real excavation has. At 6 in thick, multiply by 1.5.
Where one cubic yard becomes the line
One cubic yard is 27 cubic feet — about 81 sq ft at 4 in thick, so roughly a 9x9 slab. Below it, bagged concrete wins: suppliers have a minimum load and a short-load fee that makes a small delivery expensive per yard.
Above it the argument is not really cost, it is time. A slab has to be placed and floated before it starts to set, and a hundred bags cannot be mixed fast enough to keep a wet edge. Where the concrete you are pouring meets concrete that has already stiffened you get a cold joint, and cold joints are where slabs crack. The rest of what goes wrong.
Common questions
- How many bags for a 10x10 slab?
- 59 bags of 80 lb, or 78 of 60 lb, at 4 in thick. That is 1.23 cu yd — over the line where ready-mix is usually cheaper and much faster.
- How many for a 12x12?
- 84 bags of 80 lb, 1.78 cu yd.
- How many for a 20x20 or 30x40?
- 234 and 701 bags of 80 lb. Neither is a bag job — 14.81 cu yd is several truckloads and wants a pump or good access.
- Does a bag yield its weight in concrete?
- No. Yields are the finished volume of MIXED concrete: an 80 lb bag makes about 0.6 cu ft, a 60 lb bag 0.45, a 40 lb bag 0.3. Brands differ by a few percent, which the 5% margin absorbs.
- How thick should the slab be?
- 4 in for patios, sheds and walkways; 6 in for driveways and anything carrying a vehicle. Going from 4 to 6 in is 50% more concrete, so it changes the buy decision as well as the price.
- Do I need gravel underneath?
- 4 in of compacted gravel under the slab, yes — it drains and it gives the slab a uniform bearing surface. A slab poured straight onto graded soil cracks where the soil is soft.
Concrete calculator for a size or thickness not in the table.