How much concrete for a 20 by 20 slab?
A 20 by 20 foot slab covers 400 square feet. A very large patio, or a two car garage pad when poured 4 to 6 inches thick. At a standard thickness of 4 inches it needs 4.94 cubic yards of concrete, or 5.43 cubic yards with 10 percent extra.
20 by 20 slab at every thickness
| Thickness | Cubic feet | Cubic yards | With 10 percent extra | 80 lb bags | US tons |
|---|---|---|---|---|---|
| 3 in | 100.00 | 3.70 | 4.07 | 167 | 7.50 |
| 4 in | 133.33 | 4.94 | 5.43 | 223 | 10.00 |
| 5 in | 166.67 | 6.17 | 6.79 | 278 | 12.50 |
| 6 in | 200.00 | 7.41 | 8.15 | 334 | 15.00 |
| 8 in | 266.67 | 9.88 | 10.86 | 445 | 20.00 |
How the math works
For 4 inches, the thickness is 0.333 feet. So 20 times 20 times 0.333 is 133.33 cubic feet, and dividing by 27 gives 4.94 cubic yards. An 80 pound bag yields about 0.6 cubic feet, so the same slab needs about 223 bags.
Which thickness should I choose?
- 3 to 4 inches suits walkways, patios, and light duty pads.
- 5 to 6 inches suits driveways and garage floors with cars.
- 6 inches or more suits heavy loads, and you should ask a local builder or engineer.
Ordering tips for a pour of this size
At this size, ready mix delivery is usually easier than mixing bags. Many plants set a minimum order, so call ahead and ask about short load fees. Level and compact the base first, because a low spot adds more concrete than the flat formula predicts.
- Order 5 to 10 percent extra for uneven ground and spillage.
- Plan helpers and tools before the truck arrives.
- Check the forecast, because heat and rain both change how you pour.
These figures are planning estimates. Real material varies in moisture, size, and compaction, so confirm weights and order quantities with your supplier.
Frequently asked questions
How many yards of concrete for a 20x20 slab at 4 inches?
About 4.94 cubic yards, or 5.43 with 10 percent extra.
How many 80 pound bags for a 20 by 20 slab?
About 223 bags at 4 inches thick, using 0.6 cubic feet per bag.
How thick should a concrete slab be?
Four inches is common for patios and walks. Driveways are often 5 or 6 inches.
Should I add extra concrete?
Yes. 5 to 10 percent extra is a common allowance.