Cubic feet
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Project guide
Concrete for a Garage Slab?
How much concrete you need for a garage slab — with the right thickness, rebar, and bag-vs-truck call for a garage floor. Calculator included.
Dimensions
FIG. 01Volume ÷ 27 = cubic yards. Pros order to the nearest ½ yard and add a waste margin — running short mid-pour risks a cold joint.
You need
—cu yd
Order to nearest ½ yd → — · —
Cubic meters
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80 lb bags
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60 lb bags
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Est. weight
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Best buy
see above
Sizing it right
A garage slab is not a patio. It carries vehicle loads, so it's poured thicker and almost always reinforced — which changes both the concrete volume and what else you order.
Thickness: plan on 5–6 inches
Patios get 4 inches; a garage floor should be 5–6 inches for cars and light trucks, more if you'll park anything heavy or use a lift. That extra inch or two is real concrete: a 24×24 garage at 6 inches needs about 11.7 cubic yards versus 7.8 at 4 inches (with a 10% waste margin) — roughly 50% more. Set your thickness before you price anything.
You're in ready-mix territory
Almost any full-size garage slab is well over 2 cubic yards, so a ready-mix truck is both cheaper and far less work than bags — you'd be mixing 300+ bags by hand otherwise. Use the calculator below, then confirm with the bags-vs-ready-mix tool if you're near a small detached size.
Don't forget the rest of the order
A proper garage slab also needs a compacted gravel sub-base (4 inches typical), rebar on a grid (a 24×24 slab is roughly 750+ linear feet of #4), a vapor barrier, and usually a thickened edge — see the monolithic slab calculator if the edge carries the wall. Roll it all into one list with the cost estimator.
Worked example
A 24 × 24 ft garage at 5 inches (the right thickness for cars): 24 × 24 × 0.417 ft = 240 ft³, plus 10% waste ≈ 264 ft³, ÷ 27 =
9.78 cubic yards → order 10 yards of ready-mix. That's far past the bag-vs-truck crossover, so a truck is both cheaper and far less work. Add a 4-inch gravel base and a #4 rebar grid to the order.
Concrete by garage size (5 in thick, 10% waste)
| Garage size | Cubic yards | Order |
|---|---|---|
| 12 × 20 ft (1-car) | 4.07 | 4.5 yd |
| 20 × 20 ft (2-car) | 6.79 | 7.0 yd |
| 20 × 24 ft | 8.15 | 8.5 yd |
| 24 × 24 ft (2-car) | 9.78 | 10.0 yd |
| 24 × 30 ft (3-car) | 12.22 | 12.5 yd |
FAQ
How thick should a garage slab be?
5–6 inches for cars and light trucks, over a compacted gravel base. Four inches is fine for a patio but under-built for vehicle loads. Go thicker still for lifts or heavy equipment, and follow local code.
How much concrete for a 24x24 garage?
About 7.8 cubic yards at 4 inches, or 11.7 at 6 inches (including a 10% waste margin) — and a garage should be 5–6 inches. Enter your exact size and thickness in the calculator above for the precise figure plus bag and truck counts.