Slab
An 8 × 12 ft slab covers 96 sq ft, a size used for a pad, a step or a small patio. At 6 in thick it holds 48 cubic feet, or 1.78 cubic yards: 80 bags of 80 lb premix or 107 bags of 60 lb, before any waste.
Concrete needed
| Bag size | Bags needed |
|---|---|
| 40 lb | 160 |
| 60 lb | 107 |
| 80 lb | 80 |
| 25 kg | 117 |
| 30 kg | 97 |
Same job, other sizes
A rebar grid at 12 in on centre with 3 in of edge cover takes 20 bars, about 182 linear feet, or 10 sticks of 20 ft before laps.
Change the spacing or stick length in the rebar grid tool, or rework the volume in the slab calculator. Why every count rounds up: how many bags of concrete you need.
At this size
6 in on a 96 ft² footprint is a heavy-duty pad rather than a floor - a hot-tub or generator base, an equipment pad, anything with a point load. The extra depth over 4 in is for the load, not the span; here it costs 26 more 80 lb bags than the same pad at 4 in. Reinforcement is worth having at 6 in, and the sub-base needs to be as good as the slab.
At 8 × 12 ft this slab is small enough to pour as a single panel. Control joints go in at roughly 12-18 ft for a 6 in slab (24-36 times the thickness), and both dimensions here are inside that, so no intermediate joint is needed - just isolation material where the slab meets a wall, a step or a post.
The short side is 8 ft, so a straightedge resting on both forms strikes this off in one pass - a 10 ft screed board gives you the overhang you need at each end. Work back along the 12 ft direction and the whole 96 ft² is within reach of two people without stepping in it.
80 × 80 lb bags is 1.78 yd³ and roughly 40 mixer batches. This is the crossover point: bagging it is a full day with a mixer and at least two people, and the concrete has to keep arriving faster than the placed edge stiffens. Price 1.78 yd³ of ready-mix, short-load surcharge included, against 80 bags at your own bag price before you commit to a day of mixing.
| Length | 8 ft |
|---|---|
| Width | 12 ft |
| Thickness | 6 in |
A slab is a simple rectangular prism: multiply length × width × thickness. Enter the thickness in inches (it is usually the smallest dimension) and the plan size in feet. Four inches suits patios and walkways; drives and garage floors are usually five to six.
That volume - 48 ft³ - is divided by 27 to give 1.78 cubic yards, and by each bag's yield to give the bag count. An 80 lb bag of premix yields about 0.60 cubic feet, so this job takes 80 of them; bag counts always round up, because you cannot buy part of a bag.
Ordering ready-mix instead? Suppliers sell by the cubic yard (or cubic metre) and usually have a minimum load and a short-load surcharge - worth a call before you choose between bags and a truck.
Formwork
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