Concrete needed for a 20 × 24 ft slab, by thickness
These figures are for the whole 480 ft² slab. The last column adds 10% for waste, which is what we suggest you order.
| Thickness | Cubic feet | Cubic yards | 80 lb bags | 60 lb bags | Yards with 10% waste |
|---|---|---|---|---|---|
| 4 in | 160.0 | 5.93 | 267 | 356 | 6.52 |
| 5 in | 200.0 | 7.41 | 334 | 445 | 8.15 |
| 6 in | 240.0 | 8.89 | 400 | 534 | 9.78 |
| 8 in | 320.0 | 11.85 | 534 | 712 | 13.04 |
What a 20 × 24 slab is used for
A 20 × 24 ft slab is a standard two-car garage floor, or a big workshop. Garage slabs are commonly 4 to 6 inches thick (and thicker at the edges), with reinforcement. Follow your plans and local code.
Bags or ready-mix for 20 × 24 ft?
At 6.52 cubic yards, ready-mix is the sensible choice. Doing it with bags would mean 294 bags (about 11.8 tons of dry mix), which is too much to mix by hand before the first batch starts to set. Ask your supplier about their minimum order and how long you have to unload.
Don't forget what goes under and inside it
A slab needs a compacted base, usually 4 inches of gravel, which for this size is about 7.11 cubic yards after allowing for compaction (see the gravel calculator). Reinforcement such as rebar or wire mesh holds cracks tight: the rebar calculator counts bars for a 20 × 24 ft grid.
Want to change anything else? The calculator above is the full Concrete Calculator: edit any number and the results update instantly.
Tools & materials for this project
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Frequently asked questions
How many bags of concrete for a 20x24 slab?
For a 4 inch slab you need 267 bags of 80 lb concrete (356 bags of 60 lb), or 294 bags of 80 lb with 10% waste. For a 6 inch slab it is 400 bags of 80 lb, or 440 with 10% waste.
How many cubic yards is a 20x24 slab?
A 20 × 24 ft slab is 5.93 yd³ at 4 inches, 7.41 yd³ at 5 inches and 8.89 yd³ at 6 inches, before waste. Add 5 to 10% when you order.
How much does a 20x24 concrete slab weigh?
At 4 inches thick it weighs roughly 24,000 lb (12.0 tons), at about 150 lb per cubic foot. At 6 inches it is about 36,000 lb.
Related sizes and 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.