Concrete needed for a 20 × 20 ft slab, by thickness
These figures are for the whole 400 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 | 133.3 | 4.94 | 223 | 297 | 5.43 |
| 5 in | 166.7 | 6.17 | 278 | 371 | 6.79 |
| 6 in | 200.0 | 7.41 | 334 | 445 | 8.15 |
| 8 in | 266.7 | 9.88 | 445 | 593 | 10.86 |
What a 20 × 20 slab is used for
A 20 × 20 ft slab is the footprint of a typical two-car garage, or a very large patio or sport court. Garage floors are commonly 4 to 6 inches thick over a compacted gravel base, and are often reinforced. Check local code.
Bags or ready-mix for 20 × 20 ft?
At 5.43 cubic yards, ready-mix is the sensible choice. Doing it with bags would mean 245 bags (about 9.8 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 5.93 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 × 20 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 20x20 slab?
For a 4 inch slab you need 223 bags of 80 lb concrete (297 bags of 60 lb), or 245 bags of 80 lb with 10% waste. For a 6 inch slab it is 334 bags of 80 lb, or 367 with 10% waste.
How many cubic yards is a 20x20 slab?
A 20 × 20 ft slab is 4.94 yd³ at 4 inches, 6.17 yd³ at 5 inches and 7.41 yd³ at 6 inches, before waste. Add 5 to 10% when you order.
How much does a 20x20 concrete slab weigh?
At 4 inches thick it weighs roughly 20,000 lb (10.0 tons), at about 150 lb per cubic foot. At 6 inches it is about 30,000 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.