Slab
A 12 ft × 12 ft slab poured 4 inches thick suits a patio, walkway or shed floor. Here is the volume and the bag count worked out from the standard slab formula, length × width × thickness.
Concrete needed
| Bag size | Bags needed |
|---|---|
| 40 lb | 160 |
| 60 lb | 107 |
| 80 lb | 80 |
| 25 kg | 117 |
| 30 kg | 97 |
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 × 12 ft that is about 2 joints: 1 across the 12 ft length (2 panels of about 6 ft) and 1 across the 12 ft width (2 panels of about 6 ft). That keeps the panels close to square, which is what stops a joint being ignored and a crack appearing somewhere else. 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 12 ft direction and the whole 144 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 | 12 ft |
|---|---|
| Width | 12 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 - 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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