Concrete Slab Calculator
Find cubic yards, 60 lb and 80 lb bags, and cost for a concrete slab from length, width, and thickness. Free concrete calculator with waste.
How to use this concrete slab calculator
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.
How much does a concrete slab cost?
Ready-mix often runs $140–$200 per cubic yard before delivery, pumping, rebar, and finishing. Bags cost more per yard but win on tiny pads.
Results are estimates for informational purposes and are not financial, legal, or professional advice. Last reviewed 2026-08-20.