Concrete Slab Calculator
Everything to order for a concrete slab: concrete in yards or bags, rebar grid or welded wire mesh, chairs, and the compacted gravel base underneath.
A 12 × 12 ft slab 4 in thick needs 1.78 cu yd, about 2.0 cu yd with 10% extra (89 bags of 80 lb). A #3 rebar grid at 18 in needs about 12 sticks of 20 ft, and a 4 in gravel base about 2.1 cu yd.
Concrete for common slabs (+10%)
| Slab | Cubic yards | 80 lb bags | 60 lb bags |
|---|---|---|---|
| 10 × 10 ft × 4 in | 1.36 | 62 | 82 |
| 10 × 12 ft × 4 in | 1.63 | 74 | 98 |
| 12 × 12 ft × 4 in | 1.96 | 89 | 118 |
| 20 × 20 ft × 4 in | 5.43 | 245 | 326 |
| 12 × 24 ft × 5 in | 4.89 | 220 | 294 |
| 20 × 24 ft × 6 in | 9.78 | 440 | 587 |
Frequently asked questions
How much concrete do I need for a 12 × 12 slab?
At 4 in thick, 1.78 cubic yards, or about 2.0 with 10% extra: 89 bags of 80 lb concrete.
How much concrete for a 20 × 20 slab?
At 4 in thick, 4.94 cubic yards, about 5.4 with 10% extra. That is a ready-mix job.
How thick should a concrete slab be?
4 in for patios, walkways and shed floors; 5–6 in for driveways, hot tub pads and anywhere vehicles park.
How much gravel goes under a concrete slab?
Usually 4 in of compacted crushed stone, more on clay or wet ground. About 1.5 cu yd covers a 100 sq ft slab area at 4 in once compaction is allowed for.
How far apart should rebar be in a slab?
Commonly 12–18 in on center each way for #3 or #4 bar, set on chairs so it sits in the middle third of the slab.
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Volumes: slabs and footings length × width × depth; columns π r² h; steps width × run × rise × n(n+1) ÷ 2. Bag yields from premixed concrete labels (80 lb 0.6 cu ft, 60 lb 0.45, 40 lb 0.30). Fresh concrete weighs about 4,050 lb per cu yd. Typical thickness: 4 in for patios and walks, 5–6 in for driveways and heavier loads. Rebar laps of about 18 in; welded wire 5 × 10 ft sheets. Structural work must follow your local code.