Concrete needed for a 12 × 20 ft slab, by thickness
These figures are for the whole 240 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 | 80.0 | 2.96 | 134 | 178 | 3.26 |
| 5 in | 100.0 | 3.70 | 167 | 223 | 4.07 |
| 6 in | 120.0 | 4.44 | 200 | 267 | 4.89 |
| 8 in | 160.0 | 5.93 | 267 | 356 | 6.52 |
What a 12 × 20 slab is used for
A 12 × 20 ft slab is a one-car parking pad or a driveway section (12 ft is a standard single-lane width), or a carport floor. Vehicle loads call for 5 to 6 inches of concrete over a compacted gravel base, with reinforcement.
Bags or ready-mix for 12 × 20 ft?
At 3.26 cubic yards, ready-mix is the sensible choice. Doing it with bags would mean 147 bags (about 5.9 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 3.56 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 12 × 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 12x20 slab?
For a 4 inch slab you need 134 bags of 80 lb concrete (178 bags of 60 lb), or 147 bags of 80 lb with 10% waste. For a 6 inch slab it is 200 bags of 80 lb, or 220 with 10% waste.
How many cubic yards is a 12x20 slab?
A 12 × 20 ft slab is 2.96 yd³ at 4 inches, 3.70 yd³ at 5 inches and 4.44 yd³ at 6 inches, before waste. Add 5 to 10% when you order.
How much does a 12x20 concrete slab weigh?
At 4 inches thick it weighs roughly 12,000 lb (6.0 tons), at about 150 lb per cubic foot. At 6 inches it is about 18,000 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.