Show the math
- Convert to feet (inches ÷ 12, centimeters ÷ 30.48): Length 24 ft; Width 24 ft; Thickness 4 in = 0.3333 ft
- Volume = 24 × 24 × 0.3333 × 1 = 192 ft³
- Net volume = 192 ft³ ÷ 27 = 7.1111 yd³ (× 0.0283168 = 5.4368 m³)
- Order quantity = 7.1111 yd³ × (1 + 10%) = 7.8222 yd³ = 211.2 ft³
- 80 lb bags = 211.2 ft³ ÷ 0.6 ft³ per bag = 352 → 352 bags (always round up)
- Weight = 211.2 ft³ × 150 lb/ft³ = 31,680 lb
What this slab is used for
A 24×24 ft slab (576 ft²) is the standard detached two-car garage with extra room, a large workshop, or a generous addition. You can park two vehicles, keep a workbench and still have a path along the walls.
Five inches with steel is a sound base case for a garage of this size. Use 6 in if you expect to park heavy trucks or keep a vehicle lift, or if the soil is poor. Tie the slab into a perimeter footing or thickened edge, and install a vapor barrier under the slab if the building is enclosed.
Concrete for a 24×24 ft slab at each thickness
At 4 in the order is 7.82 yd³; at 5 in it is 9.78 yd³; and at 6 in it is 11.73 yd³. A standard mixer carries up to about 10 yd³, so 4 and 5 in fit in one truck but 6 in does not. Ask the plant whether to book two trucks or a larger delivery so the pour is continuous.
| Thickness | Net yd³ | Order yd³ | Order m³ | 80 lb bags | 60 lb bags | Weight |
|---|---|---|---|---|---|---|
| 3 in | 5.33 | 5.87 | 4.49 | 264 | 352 | 23,760 lb |
| 4 in | 7.11 | 7.82 | 5.98 | 352 | 470 | 31,680 lb |
| 5 in | 8.89 | 9.78 | 7.48 | 440 | 587 | 39,600 lb |
| 6 in | 10.67 | 11.73 | 8.97 | 528 | 704 | 47,520 lb |
The formula is 24 × 24 × thickness ÷ 12 ÷ 27 for the net volume. The order adds 10% for waste and rounds the bag counts up. Use the calculator above for another thickness or waste allowance.
Bags or ready-mix for 24×24 ft?
Bags: 352 bags of 80 lb mix at 4 in, a total of 28,160 lb of dry product. Ready-mix: 7.82 yd³, material about $978–$1,725 at the published range, last verified 2026-10-04, before delivery and short-load fees. Bagged concrete for this slab costs about $1,936–$2,816. Because the order is above two cubic yards, a truck is usually the practical choice, and at 6 in you may need 2 loads. See bagged vs ready-mix.
What to watch for
- Ordering exactly the net volume and being short at the end.
- Poor timing: on a slab this big, delays between loads create cold joints.
- Not planning the joints: aim for 12 ft panels, so one joint in each direction at the center, or 8 ft panels for tighter control.
- Failing to protect the slab during curing, which on 576 ft² of exposed surface dries fast in sun and wind.
Layout and joints
Because the slab is square, the center joints create four panels of 12×12 ft, the maximum for a 4 in slab. Add more cuts for a 4 in slab or if the concrete is likely to shrink.
For a 4 in slab, joint spacing of 8 to 12 ft is the usual range, with a 15 ft maximum for any thickness. Read the control joints guide for depth and timing.
Before you order
- Measure the forms, not the excavation, and check the thickness in several places.
- Add the volume of any thickened edge, apron or step separately.
- Plan the base: use the gravel base calculator to estimate stone for 576 ft².
- If the design calls for steel, estimate it with the rebar calculator.
- Put the quantity on the Pour List to combine it with footings and steps.
Frequently asked questions
How many yards of concrete for a 24×24 slab?
7.11 yd³ net at 4 in, 7.82 yd³ with 10% waste.
How many bags of concrete for a 24×24 slab?
352 bags of 80 lb mix or 470 bags of 60 lb mix at 4 in with 10% waste.
How much does a 24×24 slab weigh?
About 31,680 lb (15.84 US tons) at 4 in with 10% waste, using 150 lb/ft³.
How many yards for a 24×24 slab at 6 in?
11.73 yd³ with 10% waste.
How much does a 24×24 concrete slab cost in materials?
About $978–$1,725 for ready-mix concrete at 4 in, before delivery fees, based on the ranges on the cost page (verified 2026-10-04).
Sources
Estimate only. Results depend on the numbers you enter, ground conditions and your supplier. For structural work consult a licensed engineer and follow local codes. Brand names such as Quikrete and Sakrete are used only to cite published product data. This site is not affiliated with any brand.