Slab
A 15 ft × 15 ft slab poured 6 inches thick suits a driveway, garage floor or heavy-load pad. 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 | 375 |
| 60 lb | 250 |
| 80 lb | 188 |
| 25 kg | 273 |
| 30 kg | 227 |
At this size
6 in is a load thickness: repeated vehicle traffic, a garage or shop floor, a machine or skip pad, anything with a point load. At this depth reinforcement is assumed rather than optional, and the sub-base needs to be as good as the slab - a 6 in slab on soft ground still cracks.
At 15 × 15 ft this slab is small enough to pour as a single panel. Control joints go in at roughly 12-18 ft for a 6 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.
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.
188 × 80 lb bags is 4.17 yd³ - about 94 mixer batches. At this size the bag count is a costing reference rather than a plan: mixing 188 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 | 6 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 - 112.5 ft³ - is divided by 27 to give 4.17 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 188 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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