Slab
A 15 × 15 ft slab covers 225 sq ft, a size used for a patio, a shed base or a run of walkway. At 4 in thick it holds 75 cubic feet, or 2.78 cubic yards: 125 bags of 80 lb premix or 167 bags of 60 lb, before any waste.
Concrete needed
| Bag size | Bags needed |
|---|---|
| 40 lb | 250 |
| 60 lb | 167 |
| 80 lb | 125 |
| 25 kg | 182 |
| 30 kg | 152 |
Same job, other sizes
A rebar grid at 12 in on centre with 3 in of edge cover takes 30 bars, about 435 linear feet, or 22 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 15 × 15 ft that is about 2 joints: 1 across the 15 ft length (2 panels of about 7.5 ft) and 1 across the 15 ft width (2 panels of about 7.5 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.
At 15 ft across, this is past the width a screed board spans. Set 1 intermediate wet screed or screed rail down the 15 ft direction, dividing it into strips of about 7.5 ft, and strike off strip by strip. 225 ft² also has to be placed and finished before the first of it sets, so this is a crew job, not a weekend-alone job.
125 × 80 lb bags is 2.78 yd³ - about 63 mixer batches. At this size the bag count is a costing reference rather than a plan: mixing 125 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 | 15 ft |
|---|---|
| Width | 15 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 - 75 ft³ - is divided by 27 to give 2.78 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 125 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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