Slab
A 30 × 40 ft slab covers 1,200 sq ft, a size used for the floor of a building, poured as a slab on grade. At 4 in thick it holds 400 cubic feet, or 14.81 cubic yards: 667 bags of 80 lb premix or 889 bags of 60 lb, before any waste. A floor slab this size normally needs an engineered design and a building permit; the figures here are quantities only.
Concrete needed
| Bag size | Bags needed |
|---|---|
| 40 lb | 1,334 |
| 60 lb | 889 |
| 80 lb | 667 |
| 25 kg | 968 |
| 30 kg | 807 |
Same job, other sizes
A rebar grid at 12 in on centre with 3 in of edge cover takes 70 bars, about 2,365 linear feet, or 119 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 1,200 ft² this is a building floor. Its thickness, reinforcement and sub-base normally come from an engineered design, not from a rule of thumb. The table above gives the concrete for this footprint at 4, 5 and 6 in, so each option can be priced once the design sets the thickness.
A 4 in slab wants control joints every 8-12 ft - 24 to 36 times its thickness. At 30 × 40 ft that is about 5 joints: 2 across the 30 ft length (3 panels of about 10 ft) and 3 across the 40 ft width (4 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 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 40 ft direction, dividing it into strips of about 10 ft, and strike off strip by strip. 1,200 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.
667 × 80 lb bags is 14.81 yd³ - about 334 mixer batches. At this size the bag count is a costing reference rather than a plan: mixing 667 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 | 40 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 - 400 ft³ - is divided by 27 to give 14.81 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 667 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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