How this calculator works
This runs our concrete, gravel and rebar calculators on one set of measurements. The slab volume is the area times the thickness, with your waste allowance; the gravel base is the area times the base depth with 20% extra because loose stone compacts; the vapor barrier is the slab size plus a foot all round; the rebar is a grid at 18 inch centers; and the forms, stakes and isolation strip come from the perimeter.
Worked example: a 24 × 24 ft garage floor (576 ft²) poured 4 inches thick needs 7.82 cubic yards with 10% extra (7.11 without), which is 1 truck load, on about 11.9 tons of crushed stone for a 4 inch base. It takes 676 ft² of vapor barrier (1 roll), 42 twenty-foot lengths of #3 rebar, 12 form boards with 39 stakes, and 96 ft of control joints. At 6 inches it needs 11.73 cubic yards, and a 12 × 20 ft shed or workshop slab needs 3.26 cubic yards.
Concrete for common slabs (4 in, 10% waste)
Cubic yards, truck loads and base stone.
| Slab | Area | Concrete | Truck loads | Base stone |
|---|---|---|---|---|
| 10 × 12 ft | 120 ft² | 1.6 yd³ | 1 | 2.5 tons |
| 12 × 20 ft | 240 ft² | 3.3 yd³ | 1 | 5.0 tons |
| 20 × 20 ft | 400 ft² | 5.4 yd³ | 1 | 8.3 tons |
| 24 × 24 ft | 576 ft² | 7.8 yd³ | 1 | 11.9 tons |
| 24 × 30 ft | 720 ft² | 9.8 yd³ | 1 | 14.9 tons |
| 30 × 40 ft | 1200 ft² | 16.3 yd³ | 2 | 24.9 tons |
Tips before you buy
- Lay a vapor barrier over the base under any slab that will carry a floor covering, a coating or stored goods. It stops moisture rising through the concrete.
- Compact the base in layers and keep the thickness even: thin spots crack. Check the depth with a story pole before the pour.
- Cut the control joints early, within 6 to 12 hours of finishing, so that shrinkage cracks follow them.
- Keep the slab damp or covered for a week to cure slowly, and wait about a month before you seal it or put heavy loads on it.
- Plan the pour: ready-mix is on a clock, so have the forms level, the barrier and steel in, and enough hands for the finishing.
Tools & materials for this project
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Frequently asked questions
How much concrete do I need for a 24x24 garage slab?
A 24 × 24 ft slab 4 inches thick needs 7.11 cubic yards before waste and 7.82 cubic yards with 10% extra: 1 truck load. At 6 inches it is 11.73 cubic yards.
How thick should a garage floor slab be?
Four inches is the usual minimum for cars and a workshop. Use 5 or 6 inches for heavy trucks, a car lift or heavy equipment. A 24 × 24 ft slab at 6 inches takes 3.91 cubic yards more than at 4.
Does a slab need a vapor barrier?
Usually yes: 10-mil polyethylene over the base stops moisture rising through the concrete, especially if the floor will have a coating or flooring. A 24 × 24 ft slab needs 676 ft², which is 1 roll of 20 × 100 ft.
How much gravel goes under a concrete slab?
Four to six inches of compacted crushed stone. For a 24 × 24 ft slab, a 4 inch base takes about 11.9 tons, including 20% for compaction.
How many truck loads of concrete for a garage?
A ready-mix truck carries up to 10 cubic yards, so a 24 × 24 ft garage slab at 4 inches needs 1 load. Check the plant's minimum, since small orders carry a short-load fee.
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Related calculators & guides
Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.