Concrete Slab Calculator
Calculate cubic yards, 60 lb and 80 lb bags, and cost for a concrete slab from length, width, and thickness.
How the calculation works
Order concrete in cubic yards. A 4-inch slab is the usual patio and shed floor; 5–6 inches is safer for a driveway or garage. Ready-mix is cheaper than bags once you pass roughly one cubic yard. Add 5–10% waste for spillage and uneven subgrade.
This concrete slab calculator also counts bags so you can compare a DIY mix to a truck. Reinforcement (welded wire or fiber) reduces cracking but does not replace a compacted base. Local price per yard swings with fuel and cement markets.
Formula
Worked example
A 20×12 ft slab, 4 in thick, with 10% waste is 88 cu ft ≈ 3.26 cu yd (order 3.5–4 yards from a truck), or about 147 bags of 80 lb mix.
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Frequently asked questions
How many cubic yards of concrete do I need?
Length × width × thickness in feet, then divide by 27. A 10×10×4″ slab is 1.23 yards before waste.
How many 80 lb bags in a yard?
About 45 bags of 80 lb mix make one cubic yard (each bag is ~0.60 cu ft).
4 inch or 6 inch slab?
Patios and sidewalks: 4 in. Driveways and garage floors: 5–6 in with a gravel base.
Should I add waste?
Yes. 5% for a neat form, 10% if the ground is uneven or you are inexperienced.
Bags or a truck?
Bags for small pads and piers. A truck once you exceed about 1 cubic yard — mixing 45+ bags by hand is miserable.
Results are estimates for informational purposes and are not financial or professional advice.