Slab
A 12 × 12 ft slab covers 144 sq ft, a size used for a patio, a shed base or a run of walkway. At 6 in thick it holds 72 cubic feet, or 2.67 cubic yards: 120 bags of 80 lb premix or 160 bags of 60 lb, before any waste.
Concrete needed
| Bag size | Bags needed |
|---|---|
| 40 lb | 240 |
| 60 lb | 160 |
| 80 lb | 120 |
| 25 kg | 175 |
| 30 kg | 146 |
Same job, other sizes
A rebar grid at 12 in on centre with 3 in of edge cover takes 24 bars, about 276 linear feet, or 14 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 is a load thickness: repeated vehicle traffic, a garage or shop floor, a machine or skip pad, anything with a point load. At this depth reinforcement is assumed rather than optional, and the sub-base needs to be as good as the slab - a 6 in slab on soft ground still cracks.
At 12 × 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 12 ft, so a straightedge resting on both forms strikes this off in one pass - a 14 ft screed board gives you the overhang you need at each end. Work back along the 12 ft direction and the whole 144 ft² is within reach of two people without stepping in it.
120 × 80 lb bags is 2.67 yd³ - about 60 mixer batches. At this size the bag count is a costing reference rather than a plan: mixing 120 bags on site cannot keep ahead of the setting edge, so the pour ends up full of cold joints. Ordered by the yard it is part of a single load. Have the crew, the access and the discharge points sorted before anything arrives.
| Length | 12 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 - 72 ft³ - is divided by 27 to give 2.67 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 120 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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