Slab
A 12 × 20 ft slab covers 240 sq ft, a size used for a patio, a shed base or a run of walkway. At 4 in thick it holds 80 cubic feet, or 2.96 cubic yards: 134 bags of 80 lb premix or 178 bags of 60 lb, before any waste.
Concrete needed
| Bag size | Bags needed |
|---|---|
| 40 lb | 267 |
| 60 lb | 178 |
| 80 lb | 134 |
| 25 kg | 194 |
| 30 kg | 162 |
Same job, other sizes
A rebar grid at 12 in on centre with 3 in of edge cover takes 32 bars, about 464 linear feet, or 24 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
At 4 in this is a foot-traffic slab - a patio, a path, a shed floor or a garden-office base. 4 in over a well-compacted sub-base carries people and furniture indefinitely, and will take an occasional light vehicle if the base is right, but it is not a driveway thickness. Fibre or a 6×6 mesh sheet is common at this depth; the sub-base does more for the slab than the reinforcement does.
A 4 in slab wants control joints every 8-12 ft - 24 to 36 times its thickness. At 12 × 20 ft that is about 2 joints: 1 across the 12 ft length (2 panels of about 6 ft) and 1 across the 20 ft width (2 panels of about 10 ft). Those panels are longer than 1.5 : 1, which tends to crack across the middle - adding a joint on the long side to square them up is worth doing. Cut or tool them about 1 in deep - a quarter of the slab - and do it within the first day, before the slab decides where to crack on its own.
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 20 ft direction and the whole 240 ft² is within reach of two people without stepping in it.
134 × 80 lb bags is 2.96 yd³ - about 67 mixer batches. At this size the bag count is a costing reference rather than a plan: mixing 134 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 | 20 ft |
| Thickness | 4 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 - 80 ft³ - is divided by 27 to give 2.96 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 134 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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