Slab
A 30 × 30 ft slab covers 900 sq ft, a size used for a garage floor or a section of driveway. At 6 in thick it holds 450 cubic feet, or 16.67 cubic yards: 750 bags of 80 lb premix or 1,000 bags of 60 lb, before any waste.
Concrete needed
| Bag size | Bags needed |
|---|---|
| 40 lb | 1,500 |
| 60 lb | 1,000 |
| 80 lb | 750 |
| 25 kg | 1,089 |
| 30 kg | 908 |
Same job, other sizes
A rebar grid at 12 in on centre with 3 in of edge cover takes 60 bars, about 1,770 linear feet, or 89 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
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.
A 6 in slab wants control joints every 12-18 ft - 24 to 36 times its thickness. At 30 × 30 ft that is about 2 joints: 1 across the 30 ft length (2 panels of about 15 ft) and 1 across the 30 ft width (2 panels of about 15 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 1.5 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 30 ft across, this is past the width a screed board spans. Set 2 intermediate wet screeds or screed rails down the 30 ft direction, dividing it into strips of about 10 ft, and strike off strip by strip. 900 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.
750 × 80 lb bags is 16.67 yd³ - about 375 mixer batches. At this size the bag count is a costing reference rather than a plan: mixing 750 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 about 2 loads at a typical 9 yd³ truck, booked back to back. Have the crew, the access and the discharge points sorted before anything arrives.
| Length | 30 ft |
|---|---|
| Width | 30 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 - 450 ft³ - is divided by 27 to give 16.67 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 750 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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