Slab
A 18 ft × 20 ft slab poured 5 inches thick suits a residential driveway or workshop 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 | 500 |
| 60 lb | 334 |
| 80 lb | 250 |
| 25 kg | 363 |
| 30 kg | 303 |
At this size
5 in is the in-between thickness people land on when a slab has to take a car but is not a working yard - a domestic driveway, a workshop floor, a caravan or boat pad. It is a real step up from 4 in for wheel loads, and it is where reinforcement starts being worth the money rather than optional.
A 5 in slab wants control joints every 10-15 ft - 24 to 36 times its thickness. At 18 × 20 ft that is about 2 joints: 1 across the 18 ft length (2 panels of about 9 ft) and 1 across the 20 ft width (2 panels of about 10 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 1.25 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 18 ft across, this is past the width a screed board spans. Set 1 intermediate wet screed or screed rail down the 20 ft direction, dividing it into strips of about 9 ft, and strike off strip by strip. 360 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.
250 × 80 lb bags is 5.56 yd³ - about 125 mixer batches. At this size the bag count is a costing reference rather than a plan: mixing 250 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 | 18 ft |
|---|---|
| Width | 20 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 - 150 ft³ - is divided by 27 to give 5.56 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 250 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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