How Much Concrete for a 16x20 Area?
For a 16x20 area of concrete at 4 inches deep, you need about 3.95 cubic yards - roughly 178 bags (80 lb bag). The amount changes with depth; the table below shows common depths, and you can adjust everything in the calculator.
Based on a 16x20 area (320 sq ft). One cubic yard = 27 cubic feet; bags are 80 lb bag.
4 in for most slabs and patios; footings and posts are deeper - use the round option for posts.
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For a flat slab or patio, use the rectangle option with a thickness of about 4 inches. For a round footing or fence-post hole, switch to the round option and enter the hole diameter and depth.
We add a 10 percent waste allowance by default, which is standard for bagged concrete to cover spillage, uneven subgrade, and over-excavated holes. You can change it.
Other concrete amounts
Same area, other materials
Need a different size? Open the full Concrete calculator →