The dimensions used
| Length | 20 ft |
|---|---|
| Width | 20 ft |
| Thickness | 6 in |
Slab
Concrete for a 20×20 ft slab, 6 in thick
A 20 ft × 20 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
7.41cubic yards· 5.663 m³· 200 ft³
| Bag size | Bags needed |
|---|---|
| 40 lb | 667 |
| 60 lb | 445 |
| 80 lb | 334 |
| 25 kg | 484 |
| 30 kg | 404 |
Planning estimate, with no waste allowance included. Add 5–10% for spillage and over-excavation, and check the yield printed on your bag. Structural work must follow your local building code and an engineer's specification.
How it is worked out
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 — 200 ft³ — is divided by 27 to give 7.41 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 334 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
What is being measured
Nearby
More slab examples
Concrete for a 4×4 ft slab, 4 in thick0.2 yd³Concrete for a 4×4 ft slab, 5 in thick0.25 yd³Concrete for a 4×4 ft slab, 6 in thick0.3 yd³Concrete for a 4×8 ft slab, 4 in thick0.4 yd³Concrete for a 4×8 ft slab, 5 in thick0.49 yd³Concrete for a 4×8 ft slab, 6 in thick0.59 yd³Concrete for a 5×5 ft slab, 4 in thick0.31 yd³Concrete for a 5×5 ft slab, 5 in thick0.39 yd³