Slab
A 4 × 4 ft slab covers 16 sq ft, a size used for a pad, a step or a small patio. At 5 in thick it holds 6.67 cubic feet, or 0.25 cubic yards: 12 bags of 80 lb premix or 15 bags of 60 lb, before any waste.
Concrete needed
| Bag size | Bags needed |
|---|---|
| 40 lb | 23 |
| 60 lb | 15 |
| 80 lb | 12 |
| 25 kg | 17 |
| 30 kg | 14 |
Same job, other sizes
A rebar grid at 12 in on centre with 3 in of edge cover takes 8 bars, about 28 linear feet, or 2 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
5 in on a 16 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 3 more 80 lb bags than the same pad at 4 in. Reinforcement is worth having at 5 in, and the sub-base needs to be as good as the slab.
At 4 × 4 ft this slab is small enough to pour as a single panel. Control joints go in at roughly 10-15 ft for a 5 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.
12 × 80 lb bags is 0.25 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 | 5 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 - 6.67 ft³ - is divided by 27 to give 0.25 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 12 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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