Slab
A 4 ft × 4 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 | 18 |
| 60 lb | 12 |
| 80 lb | 9 |
| 25 kg | 13 |
| 30 kg | 11 |
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.
At 4 × 4 ft this slab is small enough to pour as a single panel. Control joints go in at roughly 8-12 ft for a 4 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 4 ft, so a straightedge resting on both forms strikes this off in one pass - a 6 ft screed board gives you the overhang you need at each end. Work back along the 4 ft direction and the whole 16 ft² is within reach of two people without stepping in it.
9 × 80 lb bags is 0.2 yd³ - a wheelbarrow-and-shovel job. Mix two bags at a time, keep the water tight to the bag's own figure, and place it in one go so there is no cold joint. No mixer, no delivery, no minimum load.
| Length | 4 ft |
|---|---|
| Width | 4 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 - 5.33 ft³ - is divided by 27 to give 0.2 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 9 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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