How Much Concrete for a 18×18 slab, 4" Thick?
This calculator is pre-filled to estimate the concrete for a 18×18 slab (4" thick). Change any field to match your project, and enter your local price for a precise cost.
Concrete Slab calculator
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How to estimate concrete slab
- A 4-inch slab suits patios and walkways; go to 5-6 inches for driveways and anything carrying vehicle weight.
- Ready-mix is sold by the cubic yard; for slabs over ~1 cu yd it is almost always cheaper than bagged concrete.
- Order about 5-10% extra to cover uneven subgrade and spillage. You cannot add a second batch to wet concrete.
Frequently asked questions
How many 80 lb bags are in a cubic yard?
An 80 lb bag yields about 0.6 cubic feet, so roughly 45 bags make up one cubic yard. Bagged mix only makes sense for small jobs.
How thick should a concrete slab be?
4 inches is standard for patios and shed floors, 5-6 inches for driveways and garages where vehicles park.