Show the math
- Convert to feet (inches ÷ 12, centimeters ÷ 30.48): Length 20 ft; Width 24 ft; Thickness 4 in = 0.3333 ft
- Volume = 20 × 24 × 0.3333 × 1 = 160 ft³
- Net volume = 160 ft³ ÷ 27 = 5.9259 yd³ (× 0.0283168 = 4.5307 m³)
- Order quantity = 5.9259 yd³ × (1 + 10%) = 6.5185 yd³ = 176 ft³
- 80 lb bags = 176 ft³ ÷ 0.6 ft³ per bag = 293.33 → 294 bags (always round up)
- Weight = 176 ft³ × 150 lb/ft³ = 26,400 lb
What this slab is used for
A 20×24 ft slab (480 ft²) is the most common size for a two-car garage that is more comfortable than the 20×20 minimum. It leaves room for two vehicles and a workbench along one wall. It is also a good size for a detached workshop or a studio.
Garage floors are typically 4 to 6 in. If you plan to park pickups or SUVs, use 5 in as a base case, with a thickened perimeter or grade beam. Place rebar or welded wire on chairs in the middle of the slab and keep it away from the edges. Slope the floor toward the door by about 1/8 to 1/4 in per foot.
Concrete for a 20×24 ft slab at each thickness
At 5 in the order is 8.15 yd³ with 10% waste. That is close to the typical threshold of 7 yd³ below which plants charge a short-load fee, so check whether adding a bit more concrete moves the price. At 4 in the order is 6.52 yd³. In either case this is a truck-and-crew job, and the dry weight of 294 bags alone would be 12 tons.
| Thickness | Net yd³ | Order yd³ | Order m³ | 80 lb bags | 60 lb bags | Weight |
|---|---|---|---|---|---|---|
| 3 in | 4.44 | 4.89 | 3.74 | 220 | 294 | 19,800 lb |
| 4 in | 5.93 | 6.52 | 4.98 | 294 | 392 | 26,400 lb |
| 5 in | 7.41 | 8.15 | 6.23 | 367 | 489 | 33,000 lb |
| 6 in | 8.89 | 9.78 | 7.48 | 440 | 587 | 39,600 lb |
The formula is 20 × 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 20×24 ft?
Bags: 294 bags of 80 lb mix at 4 in, a total of 23,520 lb of dry product. Ready-mix: 6.52 yd³, material about $865–$1,671 at the published range, last verified 2026-10-04, before delivery and short-load fees. Bagged concrete for this slab costs about $1,617–$2,352. Because the order is above two cubic yards, a truck is usually the practical choice. See bagged vs ready-mix.
What to watch for
- Not planning a joint layout. A 20×24 slab at 5 in needs joints roughly every 10 to 12 ft, which means two across the 24 ft side and one across the 20 ft side.
- Forgetting a vapor barrier or a stone base under an enclosed floor.
- Forgetting the garage door apron and thickened edge.
- Not having a way to reach the back of the slab. A 24 ft depth may need a pump.
Layout and joints
Order a little extra for the apron, and check whether the plant can deliver two loads in sequence if your order is near the truck capacity.
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 480 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 20×24 slab?
5.93 yd³ net at 4 in, 6.52 yd³ with 10% waste.
How many bags of concrete for a 20×24 slab?
294 bags of 80 lb mix or 392 bags of 60 lb mix at 4 in with 10% waste.
How much does a 20×24 slab weigh?
About 26,400 lb (13.20 US tons) at 4 in with 10% waste, using 150 lb/ft³.
How many yards for a 20×24 slab at 6 in?
9.78 yd³ with 10% waste.
How much does a 20×24 concrete slab cost in materials?
About $865–$1,671 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.